01 - Preheat oven to 375°F. Grease a standard 6-cup muffin tin or insert silicone muffin liners.
02 - Melt unsalted butter in a skillet over medium heat. Add minced garlic and cook for 30 seconds until aromatic. Incorporate chopped spinach and sauté until wilted, about 2 minutes. Remove from heat and allow to cool slightly.
03 - Crack eggs into a mixing bowl and whisk with whole milk, salt, black pepper, and nutmeg if desired. Blend in shredded mozzarella and grated Parmesan cheeses.
04 - Distribute sautéed spinach evenly into each muffin cup. Pour egg mixture over spinach, filling each cup approximately three-quarters full.
05 - Transfer to oven and bake for 16 to 18 minutes, until egg cups are completely set and lightly golden on top.
06 - Allow to cool for 2 to 3 minutes, then release each cup by running a knife around the edges. Serve warm.