In a large bowl combine egg, onion, 1/4 cup salsa, the chili powder, garlic, and salt. Add ground beef and mix well. Shape into a ball and place in a 9-inch pie plate. Flatten meat mixture to a 6-inch circle; set aside.
Prepare corn muffin mix according to package directions. Spread corn muffin batter over meat mixture in pie plate. Bake about 50 minutes or until an instant-read thermometer inserted into meatloaf registers 160°F
Remove from oven. Halve cheese slices and arrange on top of the corn muffin mixture. Cover loosely and let stand 10-15 minutes before serving. If desired, serve with additional salsa.