Bring first 4 ingredients to a boil in a medium saucepan; reduce heat, cover, and simmer 20 to 25 minutes or until lentils are tender.
Sauté onion and bell pepper in a large lightly greased nonstick skillet over medium-high heat until tender. Add cumin, chili powder, and garlic powder; cook, stirring constantly, 2 minutes.
Stir onion mixture and tomatoes into rice mixture, and spoon into a lightly greased 13- x 9-inch baking dish.
Bake at 350° for 15 minutes; top evenly with Cheddar cheese, and bake 5 more minutes.