First things first, preheat your oven to 375°F (190°C). This ensures the phyllo cups cook evenly and become perfectly crisp. Arrange your 24 mini phyllo shells on a baking sheet to prepare them for filling.
In a medium-sized mixing bowl, it's time to create the delicious filling. Combine the finely chopped fresh spinach, ricotta cheese, crumbled feta, shredded mozzarella, minced garlic, egg, dried oregano, and optional nutmeg. Stir everything together until it's well-combined, then season with salt and pepper to your liking.
Using a small spoon or a mini cookie scoop, carefully portion the spinach and cheese mixture into each phyllo shell. Be generous with the filling, but try not to overfill them, as this can make the cups soggy. Fill each cup just to the top.
Place the baking sheet with the filled cups into your preheated oven. Bake for 12-15 minutes. You'll know they are ready when the filling is set and bubbly, and the edges of the phyllo shells are a beautiful golden brown.
Once baked, remove the phyllo cups from the oven and let them cool for a minute or two on the baking sheet. Just before serving, sprinkle them with a little grated Parmesan cheese and some fresh, chopped parsley or dill. These are best enjoyed warm.