French Onion Frittata Recipe (with Video)

1. In a large ovenproof skillet (cast-iron works well), melt the butter and olive oil over medium-low heat. Add the thinly sliced onions.
2. Cook the onions, stirring occasionally, for about 25-30 minutes or until they are soft, deeply caramelized, and have turned a rich golden brown color. Don't rush this step; slow caramelization is key to developing the sweet onion flavor.
3. If the onions start to stick to the pan, you can deglaze it with a tablespoon or two of white wine.
4. While the onions are caramelizing, preheat your oven to 350°F (175°C).
5. In a bowl, whisk together the eggs, grated Gruyère cheese, grated Parmesan cheese, fresh thyme leaves (or dried thyme), salt, and black pepper.
6. Once the onions are caramelized, pour the egg mixture evenly over the onions in the skillet.
7. Let it cook on the stovetop for about 2-3 minutes until the edges are set.
8. Transfer the skillet to the preheated oven and bake for approximately 10-15 minutes or until the frittata is set in the center.
9. Allow the frittata to cool slightly before slicing it into wedges then serve it warm and enjoy!