French Onion Frittata

Ingredients

• 2 tbsp unsalted butter (divided)
• 2 cup day-old bread (diced)
• 1 tbsp olive oil
• 2 yellow onions (large, thinly sliced)
• 1.5 tsp kosher salt (divided)
• 1 tbsp balsamic vinegar
• 8 large eggs
• 0.25 cup whole or 2% milk (use whole or 2% milk)
• 1 tbsp Dijon mustard
• 0.25 tsp freshly ground black pepper
• 0.5 cup Gruyère cheese (diced)
• 1 tbsp fresh chives (finely chopped)

Steps

1. 1

   Arrange a rack in the middle of the oven and preheat the oven to 400°F (200°C). Use a 10-inch cast-iron skillet that can go from stovetop to oven.

2. 2

   Heat 1 tablespoon of the butter in the skillet over medium heat until it is bubbling.

3. 3

   Add the diced bread cubes. Toss them thoroughly to coat with the butter, then arrange them in a single layer.

4. 4

   Toast the bread, tossing about once every minute, until the cubes are golden-brown on all sides, about 5 minutes total. Transfer the croutons to a bowl and set them aside.

5. 5

   Reduce the skillet heat to low. Add the olive oil, sliced onions, and 1/2 teaspoon of the kosher salt.

6. 6

   Cook the onions, stirring every 5 to 10 minutes and scraping browned buildup from the bottom of the skillet, until they are very soft and deeply browned, about 40 minutes. The onions will cook down substantially and should be rich and caramelized rather than merely translucent.

7. 7

   Add the balsamic vinegar and scrape the browned bits from the bottom of the skillet so they dissolve into the onions.

8. 8

   In a large bowl, whisk together the eggs, milk, Dijon mustard, remaining 1 teaspoon kosher salt, and black pepper until evenly combined.

9. 9

   Add the remaining 1 tablespoon butter and the reserved croutons to the skillet. Stir to distribute them through the onions, then spread the mixture into an even layer.

10. 10

    Pour the egg mixture evenly over the contents of the skillet. Tilt the pan gently so the eggs settle into all the spaces.

11. 11

    Scatter the diced Gruyère evenly over the top. Cook on the stovetop until the eggs around the edges begin to set, about 2 to 4 minutes.

12. 12

    Transfer the skillet to the oven and bake until the eggs are set, about 8 to 10 minutes. Check by cutting a small slit in the center; if raw egg runs into the cut, bake for a few more minutes. Remove the frittata when the center is set.

13. 13

    Let the frittata cool in the skillet for 5 minutes. Top with the chopped chives, slice into wedges, and serve warm.

Notes

The croutons can be made up to 1 week ahead and stored in an airtight container at room temperature. The caramelized onions can be made up to 3 days ahead and refrigerated in an airtight container, or frozen for up to 3 months. Store leftover frittata in a covered container in the refrigerator for up to 5 days.

Nutrition per serving

• breakfast
• brunch
• main
• vegetarian
• comfort food
• cast-iron skillet
• stovetop-to-oven
• French-inspired