Preheat oven to 325°.
Brown ground beef and add taco seasoning and water, according to packet instructions; set aside.
In a seperate bowl, combine sour cream, 4 oz cheddar cheese, green chilies and half the onions; set aside.
Stir biscuit mix & 1/2 cup water (directions on box) to form soft dough. Pat dough on the bottom and one-half-inch up the sides of a 9x13 greased casserole dish. Will be pretty sticky, I couldn't get mine to stay up the sides!
Saute remaining onions and bell peppers until slightly tender.
On top of biscuit mix, evenly distribute ingredients in the following order: ground beef, green peppers & onion mixture, tomatoes, sour cream mixture and end with remaining cheddar cheese.
Bake for 30-40 minutes or until edges of dough are lightly browned.
You could also use a cornbread mixture on the bottom instead of the Bisquick.