Heat 1 TBS of the oil in a large skillet over medium heat. Add the onion, cover and cook, stirring until softened, about 5 minutes. Transfer to food processor.
Add the cooked lentils, toasted walnuts, brown rice, almond butter, bread crumbs, parsley, salt & pepper ( and tomato paste if using) to the onions and process to blend well. Turn the mixture out into a bowl, shape into 6 patties.
Heat the remaining 2 TBS oil in a large skillet over medium heat. Add the burger patties and cook, turning once, until well browned on both sides, about 5 minutes per side. It may be necessary to cook the burgers in batches, adding more oil as needed.
Place on the toasted burger rolls and serve hot. Enjoy!!
Alternatively you could bake these burgers in a preheated 350 degree oven for about 30 minutes.
Serving suggestion: These lentil walnut patties are great as burger sandwiches but are also very nice served as a "bunless" entree topped with sauteed mushrooms and onions or your favorite mushroom sauce and accompanied with roasted potatoes and a green salad.