In a large skillet, cook the beef until browned. Drain any grease well. Add the taco seasoning, garlic and water; simmer uncovered for 10 minutes and set aside.
Stir the chili powder into the sour cream; set aside.
Sprinkle the cornmeal into the bottom of a 9 x 13 pan. Place 6 tortillas in the bottom of the pan, being sure to overlap the tortillas AND extend up the sides of the pan slightly. If your tortillas are small, it may take more of them to make a nice layer.
Combine the two cheeses and set aside.
Spread 1 cup of salsa on the tortillas. Next, spread half of the meat mixture, followed by half of the green onions. Spread 1 cup of sour cream on top of that, followed by 2 cups of the mixed cheese.
Repeat the layers, beginning with the tortillas.
Place another layer of tortillas on the top, followed by the remainder of the salsa.
Bake at 375ºF for 40 minutes. Sprinkle the remaining 2 cups of shredded cheese on top, and bake an additional 10 minutes, or until the cheese is nice and melted.
Let stand for 10 minutes before serving.