Add lentils and 1 teaspoon of salt to 3 cups of boiling water. Reduce heat to medium-low and simmer, uncovered until lentils are tender, about 20 minutes. Drain cooked lentils in a wire-mesh strainer. Spread drained lentils on a triple paper towel lined large-rimmed baking sheet. Blot lentils with additional paper towels to absorb as much of the liquid as possible. Set aside to cool.
Add 2 cups of water to a microwave-safe container like a large measuring cup. Heat the water to boiling in the microwave. Remove from microwave. Add ½ teaspoon of salt and the bulger; stir to combine. Cover and let stand until the water is absorbed, about 15 minutes. Drain in wire-mesh strainer, pressing excess liquid out with the back of a spoon. Set aside to cool.
Heat 1 tablespoon of oil in a large non-stick skillet over medium-high heat. Add onions, celery, leeks and garlic. Cook, stirring occasionally until vegetables begin to brown, about 10 minutes. Spread vegetable mixture on a large-rimmed baking sheet to cool.
Heat remaining 1 tablespoon of oil over medium-high heat in the same skillet used to cook the onion mixture. Add the mushrooms. Cook, stirring occasionally until the mushrooms brown and the liquid is evaporated, about 10 to 12 minutes. Spread cooked mushrooms on baking sheet with onion mixture. Cool to room temperature, about 20 minutes.
Pulse process cashews in food processor fitted with a steel blade until finely chopped, about 15 pulses.
Transfer lentils, bulgur, onion mixture, and mushrooms to a large bowl. Add mayonnaise and finely chopped cashews. Stir until combined.
Transfer half of burger mixture to food processor fitted with steel blade. Pulse process until coarsely chopped, about 15 pulses. Transfer to a large bowl. Repeat with remaining burger mixture; combine with the first batch.
Add Panko breadcrumbs, remaining 1 ½ teaspoons of salt and pepper. Stir to combine. Correct seasonings. (See Tip 4)
Divide mixture into 12 equal portions, about 4.4 to 4.5 ounces each. shape into patties that are roughly 4-inches in diameter and ½-inch thick. May be covered and refrigerated for several days or frozen at this point.
Heat 1 to 2 tablespoons of oil in a large non-stick skillet over medium heat. Add 2 to 4 burgers. Cook until golden brown, about 7 minutes. Flip and cook the other side until golden brown, about 7 minutes.
Serve as desired.
Yield: 12 Tasty Meatless Burgers.