Preheat oven to 375 degrees. Lightly grease baking sheets. Cook spinach according to package directions; drain well. Transfer to large bowl. Add Parmesan and onion. Blend in eggs. Stir in croutons and butter. Shape mixture into 1-inch balls. (Can be prepared 1 week ahead, covered and frozen. Do not thaw before baking. Just increase baking time to 20-25 minutes.) Arrange on prepared baking sheets, spacing 1/2" apart. Bake until golden brown, about 15 minutes.