Preheat the oven to 350°F (175°C) and prepare a baking sheet by lining it with parchment paper or greasing it lightly with olive oil.
Thoroughly wash the spinach and steam it for about 3-5 minutes until wilted. Once cooked, let it cool slightly and then squeeze out excess moisture. Chop the spinach finely.
In a large mixing bowl, combine the chopped spinach, breadcrumbs, eggs, Parmesan cheese, chopped onion, minced garlic, salt, black pepper, and red pepper flakes (if using). Mix well until all ingredients are fully incorporated.
Using your hands, shape the mixture into small balls, about the size of a golf ball. Place the formed spinach balls on the prepared baking sheet, leaving some space in between.
Bake in the preheated oven for 20-25 minutes, or until the spinach balls are golden brown and firm to the touch.
Remove from the oven and let cool slightly before serving.