Place lentils, 1 teaspoon salt, and 3 cups of water into a saucepan and bring it to a boil over medium-high heat. Once boiled, reduce heat to medium-low and let it simmer, uncovered, for about 25 minutes. Stir it occasionally. Strain it through a fine-mesh strainer, transfer to a bowl and set it aside to cool.
Meanwhile, place the vegetable stock and 1/2 teaspoon salt in a saucepan and bring it to a boil. Stir in the bulgur, cover the lid and take it off the heat. Let it sit for 20 minutes until all the water is absorbed by the bulgur. Drain it in a fine mesh strainer to get rid of the excess water, if there is any left. Place it in a small bowl and set it aside.
Heat 1 tablespoon of olive oil in a large non-stick skillet pan over medium heat and add onions, celery, and garlic. Stirring occasionally, until the vegetables are softened and lightly browned, for 10 minutes. Off the heat, let it cool for 10-15 minutes. Wipe the skillet with a paper towel, no need to wash.
Heat 1 tablespoon of olive oil, in the empty skillet over high heat. Add the mushrooms and cook, stirring occasionally, for 10 minutes or until they are softened. Drain and set aside and cool for 10 minutes.
While lentils, bulgur wheat, and veggies are cooling, place the cashews in a food processor. Process them until they are finely chopped, for 12-15 pulses. Do not over pulse, they may turn into cashew butter. Do not wash the bowl of the food processor.
Place lentils, bulgur, vegetables (onion mixture and mushrooms), cashews, bread crumbs, vegan mayo, 1 teaspoon kosher salt, and ½ teaspoon black pepper. Give it a mix.
Place ¼ of the mixture into the food processor and pulse until it is coarsely chopped, between 15 and 20 pulses, stopping the machine and scraping the sides of the bowl a few times. Transfer it to a large bowl and repeat the same process for the rest of the mixture.
Divide it into 12 portions (about ½ cup each) and shape them into burgers, about 4 inches in diameter. (I used my clean hands to do this) Place them on a baking dish lined with parchment paper.
To cook, heat 2 tablespoons of olive oil in a 12-inch non-stick skillet over medium-high heat. Cook 3-4 burgers at a time, 4 minutes on each side until they are brown. Repeat the same process for the rest of the burger patties.
As you are cooking the burgers, you can put them in low heat (250 F degrees) oven to keep them warm. I warmed the burger buns in the oven while I was cooking the burger patties as well.
To build your burger, place a slice of tomato on a bottom burger bun. Top it off with a bulgur wheat patty, salad greens, and sliced red onion. If using garnish it with my cucumber yogurt or tahini sauce. Put the top bun on and serve.