Butter 6 slices of bread; place the bread butter-side-down in a 9x13-inch baking dish. Sprinkle with cheddar cheese. Place the ham on top of the cheese; then layer with mushrooms and chiles. Top with Monterey Jack cheese.
In a bowl, beat the eggs and add milk, salt, paprika, basil, dry mustard, onion salt, and pepper. Pour over the ingredients in the dish. Cover with plastic wrap and refrigerate overnight.
To bake: Preheat the oven to 325°F. Uncover the casserole and bake for 50 to 60 minutes, or until the eggs are set and no longer liquidy. Let stand for 10 minutes to set before serving.
Notes
*Ham is pretty salty on its own, so go lightly if you choose to add salt to the recipe.
*You can go a little lighter on the cheese if you prefer.