Here at Foolproof Living, we love roasting vegetables. If you feel the same way, be sure to also check out our Roasted Eggplant, Roasted Asparagus, and Roasted Butternut Squash recipes and add them into your weekly meal prep.

Roasted red onion slices in a bowl with a spoon on the side.

Ingredients

You only need a little oil and a pinch of salt and pepper to bring out the flavors while cooking red onions to get that gorgeous caramelization. Here’s what you’ll need:

Red onions: When you’re at the grocery store, look for large onions that are free firm and free of bruises with their peppery skin still intact.
Extra virgin olive oil: I like to use extra virgin olive oil (or another oil with a high smoke point) to get that nice caramelization for your roasted onion slices. Another good option that can withstand high cooking temperatures is avocado oil.
Seasonings: The beauty of this recipe is that you only need a tiny bit of seasoning to get a lot of flavor. Kosher salt and black pepper will do the job of bringing out the natural sweetness of red onions.

Optional Add-Ins

You may love the taste of your roasted red onions using only the simple ingredients above. However, you can incorporate other ingredients in many ways to make this recipe work for whatever you’re cooking it with. Here are some options below:

Garlic Powder: Like garlic, onions are part of the allium family. Season the onions with a dash or two of garlic powder for a savory note.
Balsamic vinegar: Tossing them with a tablespoon of balsamic vinegar is an easy way to infuse them with even deeper flavor, taking this simple recipe to the next level. Balsamic roasted red onions are especially ideal for topping salads.
Spices: You can use a variety of spices for your baked red onion recipe, depending on what you’re making them with. Dried thyme, oregano, and rosemary are good choices for savory dishes.
Brown Sugar: It is optional, but you can add a teaspoon of brown sugar (or coconut sugar) if you want sweeter onions. The sugar will enhance the caramelization and help release more of its juices.
Fresh Herbs: I love finishing a dish with fresh herbs. The brightness of the herbs balances the deep roasted taste of the onions. Try fresh thyme, dill, parsley, or anything else you like.

How To Cook Red Onions in The Oven?

Cooking red onions is so quick, you’ll wonder how you’ve never tried it before. Your new favorite topping will be ready to serve on the dinner table in less than half an hour. Here’s the best way to make this tasty dish:

Preheat the oven: To start, preheat the oven to 400 degrees. Line a rimmed baking sheet with parchment paper, and set aside.

Red onions cut and sliced on a cutting board.

Cut the onion: You have two options when cutting the onion. One is to slice them into rings and keep them whole. The other option is to slice them into rings, cut them in half, and separate them. Remember to peel off the papery skin before slicing. I recommend cutting them into ¼-inch-thick slices for roasting.

Red onion slices cut and seasoned according to the way they were cut.

Season the onions based on how you cut them:
- Cut into rings: If you slice them into rings, place them evenly on your baking sheet. Use a brush to lightly coat them with extra virgin olive oil on each side, and sprinkle them with salt and pepper (optional).
- Thinly sliced: Separate the red onion slices and place them in a medium-sized bowl. Drizzle with olive oil and sprinkle with salt and pepper. Gently toss them together. Spread them on your sheet pan in a single layer.

Baked red onions after they came out of the oven.

Roast: Place your sliced red onions in the oven. For rings, flip them halfway through, or give them a good stir for thin slices. Cook for about 20 minutes until soft and turned lightly golden brown.
Serve: Take them out of the oven, and let them cool for a few minutes before using them in your recipe.

Storing and Reheating Instructions

We love the fact that baked red onion is a condiment that stores well. Here’s how we store and reheat leftovers:

Storing: Let your oven-roasted sliced onions cool to room temperature. Keep them fresh in an airtight container and refrigerate for 3-4 days.
Reheating: You can enjoy this recipe either hot or cold, but if you’d prefer it hot, you can reheat it in three ways.
- Oven: Put the roasted onions in an oven-proof baking dish with a splash of water. Roast them in preheated 350 degrees F. oven until heated through for about 8-10 minutes.
- Sautee: Add the red caramelized onions to a saucepan with oil and sautee until thoroughly warmed.
- Microwave: Place onions in a microwave-safe container with a splash of water. Heat in 30-second increments, stirring each time, until heated through.

How to Use Roasted Red Onions in Your Daily Cooking?

There are countless ways you can use this condiment. The real question is, what don’t they go with? Here are a few uses for red onions:

Salads: Caramelized onions pair beautifully with salads, especially if you use cheese in your salads. This La Scala Salad recipe is a prime example. The sweet tang from the onions is absolutely heavenly with feta or goat cheese. You can use butter lettuce or spring mix like in my Goat Cheese and Beet Salad. Another option is to replace the raw red onions in this Friends Salad recipe with cooked red onions.
Pizzas & Taco Topping: Roasting onion to add on top of pizza will please any crowd. They’re also a great topping option for taco recipes like my Black Bean, Quinoa Tacos, and Beef Brisket Tacos.
Burgers, Sandwiches, and Wraps: Red onion is a staple for burgers, sandwiches, and wraps, so why not swap them out with its roasted version? Sink your teeth into a Guacamole Bacon Burger or Greek Pita Wraps topped with roasted red onion slices!

Oven roasted red onions on a plate with a fork on the side.

Expert Tips:

While the recipe for roasted red onions is pretty straightforward, there are a few things that we’d like to mention so that you can get the best results on your first try:

Slice them evenly: It is important that the onion slices are cut evenly (about ¼-inch thickness) to ensure even baking. You can cut them thicker but again, make sure they are even in size.
Coat with oil and season well: Take your time when coating the slices with olive oil and seasoning them with salt and pepper. This is an important step in making sure that they are perfectly caramelized and ready to use in recipes.
Do not overcrowd the pan: It is important to spread them on a single layer when baking. If you are making this recipe for a crowd (using more onions), we recommend using two baking sheets or cooking them in batches.
Flip/Rotate: Do not forget to stir (or flip) the onion slices as they are roasting. Doing so ensures even browning and prevents them from burning.
Customize: Feel free to make it your own and customize it to fit the dish you intend to incorporate them into. Enhance the flavors by using various spices and fresh herbs to complement your meal.

Roasted red onion slices are in a casserole dish from the top view.

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