Place lentils, 1 teaspoon salt and 3 cups of water in to a saucepan and bring it to a boil. 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. Line 2 layers of paper towels on a sheet pan and spread lentils on paper towels. Pad-dry them with paper towels. Let it dry at room temperature.
While lentils are simmering, 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 mash strainer to get rid of the excess water. Place it in a small bowl and set aside.
Heat 1 tablespoon of olive oil in a large non-stick skillet pan over medium heat and add onions, celery, leek and garlic. Stirring occasionally, until the vegetables are brown, for 10 minutes. Spread the cooked vegetables on a second baking sheet and let them cool in room temperature.
Heat 1 tablespoon of olive oil, in the empty skillet over high heat. Add the mushrooms and cook them for 10-12 minutes until they are brown. Stir it occasionally. Once cooked, spread them on the same baking sheet as vegetables. Let them cool to room temperature for 20 minutes.
While they are cooling, place the cashews in a food processor. Process them until they are finely chopped, for 12-15 pulses. Do not overpulse, they may turn into cashew butter. Do not wash the bowl of the food processor.
Stir the cashews into bulgur and mix well.
In a big bowl, stir in the vegetables (with mushrooms), cooked lentils, bulgur (with cashews) and vegan mayo.
Place half of the mixture into the food processor and pulse until it is coarsely chopped, between 15 and 20 pulses. Transfer it to a large bowl and repeat the same process for the second half.
Stir in Panko or breadcrumbs, 1 teaspoon salt and black pepper. Mix well. Line a baking dish with paper towels. Divide the mixture to 12 portions, 1/2 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 paper towels. (Paper towels help absorb the excess moisture.)
To cook: Heat 2 tablespoons of olive oil in a 12-inch non-stick skillet over medium high heat. Cook 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 a 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.
Build your burger using, lettuce, kale, tomatoes, zucchini…etc. Anyway you like!