Preheat oven to 450 degrees.
Line a sheet pan with parchment paper.
Place bagel halves, cut side up, in a single layer on the parchment paper.
Butter each bagel half. Crack one egg into the center hole of each bagel half.
Top each bagel half with two tablespoons of spinach, two tablespoons of cooked bacon and 1/4 cup of shredded mozzarella cheese. Season to taste with salt and pepper.
Bake bagels for 10 - 15 minutes, or until egg yolk is cooked through.
Optional Topping Ideas: Ham, chopped green pepper and onions, cheddar cheese; Cooked mushrooms, Gruyere cheese; Sausage, chopped artichoke hearts, provolone cheese; Tomato sauce, pepperoni, mozzarella cheese