Preheat oven to 350°F (180°C). Line baking sheet with parchment paper. In large skillet, heat butter over medium heat; cook onion and garlic until softened, approximately 5 to 7 minutes. Add kale, spinach, salt and pepper; cook until wilted, approximately 5 minutes. Transfer to large bowl; let cool completely.
Stir in parsley, lemon juice and Worcestershire sauce. Stir in Fontina, Parmesan and eggs. Add breadcrumbs; mix well until mixture holds together (mixture will be a little wet.) Shape into 1-inch (2.5 cm) balls. Place on prepared baking sheet.
Bake until firm to the touch and golden brown, approximately 30 minutes. Let cool for 5 minutes before serving.