Preheat oven to 375°F. Line a baking sheet with parchment.
Squeeze the thawed spinach very dry using a clean towel.
Whisk the beaten eggs and cooled melted butter in a large bowl.
Stir in Parmesan, mozzarella, garlic powder, onion powder, Italian seasoning, salt, pepper, and nutmeg.
Fold in the spinach until evenly distributed.
Add breadcrumbs and mix until the mixture holds together and feels slightly tacky.
Chill the bowl for 10 minutes to firm the mixture for easier rolling.
Scoop tablespoon portions and roll into balls. Arrange on the prepared sheet.
Bake 14–16 minutes, rotating the pan once, until lightly golden and set.
Cool 5 minutes on the sheet, then serve warm.