Saute meat, beans, onions, taco seasoning, sauce, and corn on medium-high heat until meat is browned (or soy crumbles are thoroughly cooked).
Spoon half your meat mixture into your casserole dish in an even layer, top with a thin layer of sour cream, then sprinkle with some cheese.
Repeat. Cover with cheese.
Bake at 350 for 15 minutes, then serve topped with diced tomatoes and green onions.