1. Preheat your oven to 375°F (190°C) and grease a 12-cup muffin tin with olive oil or cooking spray.
2. In a medium skillet, heat the olive oil over medium heat. Add the chopped onion and cook until translucent, about 5 minutes.
3. Add the minced garlic and cook for another minute until fragrant. Stir in the spinach and cook until wilted, about 3 minutes. Set aside to cool slightly.
4. In a large mixing bowl, whisk together the eggs, milk, Parmesan, and mozzarella cheese. Season with salt, pepper, and nutmeg, if using.
5. Fold the spinach mixture into the egg mixture until well combined. If using, gently fold in the cherry tomatoes.
6. Divide the mixture evenly among the muffin cups, filling each about three-quarters full.
7. Bake in the preheated oven for 20-25 minutes, or until the muffins are set and lightly golden on top. A toothpick inserted into the center should come out clean.
8. Allow the muffins to cool in the pan for a few minutes before transferring them to a wire rack to cool completely.

As you prepare these muffins, remember that cooking is an art, not a science. Don’t be afraid to make adjustments and let your creativity shine, just as my grandmother taught me.