Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

Melt 2 tablespoons of unsalted butter in a large pan over medium-high heat. Add chopped onions and sauté until translucent, about 3-4 minutes. Stir in minced garlic and cook for an additional minute.

In a mixing bowl, combine thawed spinach, breadcrumbs, beaten eggs, grated Parmesan cheese, salt, and pepper. Pour in sautéed onion and garlic mixture and mix until well combined.

Form the mixture into round balls, about 1 inch in diameter, and place them on the prepared baking sheet.

Bake for 15 to 20 minutes until lightly browned and crispy on the outside. Let them cool slightly before serving.