Brown your meat with the yellow onion, crumbling as it cooks. While that's browning combine all ingredients for the taco seasoning; set aside.
Drain the meat and return to the pan. Add the taco seasoning and water and stir well. Stir in the tomato sauce and allow to simmer for about 5 minutes.
Cover the bottom of a medium size casserole dish with 1/2 of the crushed taco shells.
Layer half of the meat mixture on the taco shells. Top with half of the cheese.
Repeat the layering process and place in the oven for 20 minutes.
Serve with lettuce, tomatoes, salsa, sour cream etc.. (Whatever you normally eat on your tacos.)