In a large mixing bowl, whisk together the eggs, egg whites, Dijon mustard (if using), salt, and pepper until well combined.
Add the chopped ham, shredded cheese, chopped tomatoes, spinach, and green onions to the egg mixture. Stir to distribute the ingredients evenly.
Pour the egg mixture into the muffin cups, filling each about two-thirds full.
Place the muffin tin in the preheated oven and bake for 20 minutes, or until the egg muffins are set, puffy, and lightly golden on top.