1. Preheat oven to 425°.
2. Lightly coat both sides of bread with cooking spray. Press each bread slice into the cup of a muffin tin. Bake at 425° for 10 minutes or until bread is lightly toasted. Cool slightly.
3. Reduce oven temperature to 350°.
4. Combine milk and eggs, stirring well with a whisk. Divide the egg mixture evenly among bread cups. Sprinkle onions, if desired, and bacon evenly over tarts; top each tart with 1 tablespoon cheese. Bake at 350° for 15 minutes or until set.