Reduce heat to low, then add chili powder and mild green chilies. Add 3 cups of cheese and stir 30 seconds, or just until cheese melts and sauce is smooth. Remove from heat. If pasta has stuck together, stir quickly to separate the pieces. Spread half in baking dish, then add half the sauce over and mix gently to coat the pasta evenly.
Spoon tomatoes on top, then spread remaining pasta over the tomatoes.
Spoon remaining sauce evenly over pasta. Bake for 30 to 35 minutes, until top is golden brown and sauce is bubbling. Recipe courtesy of the California Milk Advisory Board. In a large heavy bottom pot, heat oil and saut onion for 5 minutes, adding garlic after 3 minutes. Add meat, breaking it up with a fork to brown.