1) Combine lentils, bread crumbs, egg whites, spinach, carrot, onion, Cheddar cheese, salt and pepper in a bowl. Shape into 6 equal patties. Place on a baking sheet. Cover and refrigerate for 2 hours.
2) Heat the oil in a large nonstick skillet. Add burgers; cook 6 minutes, turning over once halfway through cooking.
3) Meanwhile, if preparing the mayonnaise, mix together mayonnaise, mustard and catsup in a small bowl.
4) Place burgers in buns and serve with mayonnaise if using.
Nutrient Value Per Serving: 424 calories, 24 g protein, 11 g fat, 60 g carbohydrate, 918 mg sodium, 20 mg cholesterol.
Exchanges: 3 and half starch/bread, 1 and half meat, one third vegetable, 1 and quarter fat.