Spray a small rectangular or square glass baking dish. Set aside. Cook onion in 1 teaspoon butter in microwave. In a large bowl, mix eggs, milk, bread cubes, cooked onions, and cheese. Generously sprinkle spinach on bottom of baking dish. Pour egg mixture over top and refrigerate for several hours to overnight.
When ready to bake, preheat oven to 350 degrees F. Bake for 40 minutes. Halfway into baking, sprinkle egg mixture with additional cheese, as desired.
Can replace pepper with red pepper flakes for added spice