Spray bottom of 13 x 9-inch baking dish with cooking spray. Place bread slices in the bottom of the dish, do not overlap them. Place the butter in little pats on top of each bread slice.
Cook meat/meatless product according to package directions or how you prefer (meatless sausage patties can be placed in the microwave for 40 seconds). Saute mushrooms in oil in pan over medium heat until lightly brown. Place ground up meat/meatless product in pan with mushrooms. Sautee for another 2-5 minutes, just until flavors are melded.
While that is cooking, whisk eggs, milk and chili powder in large bowl. Stir meat and mushroom mixture in with the egg mixture. Pour over bread slices in pan, spread it out so that it evenly covers the bread. Sprinkle cheese on top. Cover with plastic wrap and place in the refrigerator overnight.
Preheat oven to 350 degrees F. Place casserole in oven and bake 45 minutes.