Simmer lentils in 4 cups water in a saucepan over medium heat 20 minutes or until tender; drain, reserving 1 cup liquid.
Sauté onion and bell pepper in hot oil in a large skillet over medium-high heat 5 minutes or until tender. Remove from heat; stir in lentils and cheese. Spoon mixture evenly down center of each tortilla, and roll up. Place in a 13- x 9-inch baking dish.
Bring 3/4 cup reserved liquid, tomato sauce, cumin, and taco sauce to a boil in a saucepan. Stir together remaining 1/4 cup reserved liquid and cornstarch until smooth. Stir into sauce mixture, and cook over medium heat, stirring often, 10 minutes. Pour over burritos.
Bake at 350° for 10 minutes or until thoroughly heated. Serve with desired toppings.