In a medium saucepan place lentils and cover with one inch of water. Bring to a boil, reduce heat to low and simmer covered for 25 to 35 minutes or until tender. Drain the lentils, rinse, and set aside. Cook the rice according to the package directions and set this aside also.
Using a heavy large skillet heat the oil over medium heat. Add the mushrooms and chopped onion cooking and stirring for 20 to 25 minutes or until mushrooms are brown. In a large bowl combine the mushroom and onion mixture with the cheese, walnuts, lentils, rice, cliantro, eggs and pepper, mix together well. Cover the bowl and chill.
Preheat broiler and grease a 15 x 10 inch jelly roll pan with oil. Take lentil mixture out of refrigerator and shape into six 1/2 inch thick patties. Place patties on oiled jelly roll pan and broil four inches away from heat source for 3 to 4 minutes on each side, turning once, or until golden brown. Serve immediately on buns topped with mayo, red onion, lettuce, and tomatoes.