Cook onion, green pepper and garlic in hot olive oil for 5 minutes. Cool.
In a large bowl, combine the cooled veggies with the lentils, rice, tomatoes and seasonings. Stir to combine.
Generously scoop filling into tortillas; sprinkle optional cheese on top of the filling, if desired. Roll up tortillas, place in a casserole dish. (If making this ahead of time, store in the refrigerator at this point.)
Place casserole dish in a 350 degree oven and cook until heated through, about 15 minutes. Top with sour cream, if desired.