Preheat oven to 400 degrees; coat a jumbo muffin pan with cooking spray, see shopping tip.
Roll the 6 muffin halves a bit larger, using a rolling pin to flatten. Place a muffin half in each muffin tin, pressing up the sides to form the baskets. It doesn’t need to go all the way to the top of the tin.
Combine the melted butter and mustard in a small bowl. Using a pastry brush, brush butter mixture all over the inside of the cups. Place in oven and toast until slightly browned, about 10-12 minutes.
In the meantime, in a large nonstick pan, heat the olive oil. Add onions, garlic and saute over medium heat until softened about 3-4 minutes. Add spinach and cook until just starting to wilt. Stir in tomatoes. Remove from heat and season to taste with a little salt and pepper.
In a small bowl, whip together the eggs and egg whites to combine. Add eggs among 6 toasted baskets, about ¼ cup for each bread cup. Divide spinach mixture among the 6 cups and top each with 1 tablespoon of cheese.
Bake in oven for about 15 minutes or until eggs are cooked. Remove from oven and lift baskets from pan with a knife.
Serve immediately with a fork and knife.