Fry sausage and onion over medium heat until sausage is browned. Preheat oven to 375 degrees. Pour 1/2 can enchilada sauce into greased 9 x 13 baking dish. Lay 6 tortillas to cover the bottom of the dish. Cover with 1/2 can of the enchilada sauce. Top with 1/2 of the sausage mixture and all of the green chilis. Spread all of the sour cream on top of the chilis. Sprinkle with 1/3 of the cheese. Add another layer of 6 tortillas. Top with 1/2 can of enchilada sauce, remaining sausage, and 1/3 of cheese. Top with remaining 6 tortillas, sauce, and cheese. Cover with foil and bake 20 minutes. Uncover and bake for an additional 10 minutes or until cheese is lightly browned.