Trim the cabbage and give it a rough chop. Cook in boiling salted water until softened about 10 minutes. Drain well. Place in bottom of 13x9 baking pan.
Add butter to skillet over medium heat and cook onion, garlic, and red pepper until softened. Add flour and stir to make a roux. Add the milk and stir until thickened. Season with salt. Pour over cabbage. Bake at 375 for 25 minutes.
Combine the mayonnaise , cheddar cheese, and salsa. Spread over top of cabbage and bake another 10 minutes.