Preheat oven to 350°. Spray muffin pan with nonstick cooking spray.
Cook bacon in large skillet on medium-high heat until crisp. Drain, cool and crumble.
In same skillet, cook and stir onion in remaining bacon fat 5 minutes or until tender. Ad spinach, pepper, nutmeg and salt. Cook and stir over medium heat about 3 minutes or until liquid evaporates. Return bacon to skillet and remove from heat.
Combine ricotta, mozzarella and Parmesan cheeses in large bowl; mix in eggs. Add cooked spinach mixture and combine well.
Divide mixture evenly to fill 10 muffin cps. Bake 40 minutes or until filling is set. Let stand 10 minutes. Run thin knife around edges to release. Serve hot or refrigerate and serve cold.