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1
Bring 3 cups water, lentils, and 1 tsp salt to boil in medium saucepan over high heat; reduce heat and simmer, uncovered, stirring occasionally, until lentils are just beginning to fall apart, about 25 minutes. Drain and dry. Cool lentils to room temperature.
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2
While lentils simmer, bring 2 cups water and 1/2 tsp salt to boil in small saucepan. Stir bulgur wheat into boiling water and cover; let stand off heat until water is absorbed, 15-20 minutes. Drain in fine-mesh strainer; press out excess moisture. Transfer bulgur to medium bowl and set aside.
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3
Heat 1 tbsp oil in 12-inch nonstick skillet over medium-high heat. Add onions, celery, leek, and garlic; cook, stirring occasionally, until vegetables begin to brown, about 10 minutes. Spread vegetable mixture on second baking sheet to cool; set aside. Add remaining 1 tbsp oil to skillet; heat then add mushrooms and cook, stirring occasionally, until golden brown, about 12 minutes. Spread mushrooms on baking sheet with vegetable mixture; cool to room temperature.
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4
Process cashews in food processor until finely chopped; stir into bulgur along with cooled lentils, vegetable-mushroom mixture, and mayonnaise. Transfer half of mixture to food processor and pulse until coarsely chopped; mixture should be cohesive but roughly textured. Transfer mixture to large bowl; repeat with remaining mixture. Stir in panko, 1 teaspoon salt, and pepper to taste.
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5
Line baking sheet with paper towels. Divide mixture into 12 portions, about 1/2 cup each; set patties on baking sheet. (Patties can be covered with plastic wrap and refrigerated up to 3 days.)
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6
Grill burgers, without moving them, until well browned, about 5 minutes; flip burgers and continue cooking until well browned on second side, about 5 minutes.
