Slice the onions thinly. In a skillet, melt butter over medium heat, add onions, and sauté until translucent and caramelized, about 10-15 minutes.

Combine sautéed onions with cheddar and Parmesan in a mixing bowl. Add the melted butter, flour, salt, and pepper. Mix until it forms a cohesive dough.

Preheat oven to 375°F (190°C). Line a baking sheet with parchment paper. Scoop the mixture onto the sheet, flattening each scoop into ¼ inch rounds.

Bake in preheated oven for 20-25 minutes until golden brown and crispy.

Remove from oven and cool on a wire rack for a few minutes before serving.