Combine rice, lentils, sweet potato, spinach and mushrooms in a large bowl. Add soy sauce, salt and pepper. Add bread crumbs and cilantro and mix well. Refrigerate for 15 to 39 minutes. Form mixture into 6 to 8 patties. Oil a vegetable grill generously--these patties tend to stick. Grill for 3 to 4 minutes on each side, until medium brown. Makes 6 to 8 patties. Variation: Instead of burgers, form mixture into small balls. Grill until browned. Serve as a snack with a dipping sauce or add to pasta with your favorite sauce.