01 - Preheat oven to 375°F and lightly grease a standard 6-cup muffin tin or line with silicone liners.
02 - Melt butter in a skillet over medium heat. Add the minced garlic and sauté for 30 seconds until aromatic.
03 - Stir in the chopped spinach and cook until wilted, about 2 minutes. Remove from heat and allow to cool slightly.
04 - In a large mixing bowl, whisk eggs, whole milk, salt, black pepper, and ground nutmeg until well blended. Incorporate the mozzarella and Parmesan cheeses.
05 - Evenly distribute sautéed spinach among muffin cups. Pour the egg and cheese mixture over spinach, filling each cup to approximately three-quarters full.
06 - Bake in the preheated oven for 16 to 18 minutes, until egg cups are set and lightly golden.
07 - Cool for 2 to 3 minutes. Run a thin knife around the edges to release egg cups. Serve warm.