Ingredients
2 tablespoons unsalted butter, divided
2 cups diced day-old bread
1 tablespoon olive oil
2 large yellow onions, thinly sliced
1 1/2 teaspoons kosher salt, divided
1 tablespoon balsamic vinegar
8 large eggs
1/4 cup whole or 2% milk
1 tablespoon Dijon mustard
1/4 teaspoon freshly ground black pepper
1/2 cup diced Gruyère cheese (2 ounces)
1 tablespoon finely chopped fresh chives
Instructions
- Preheat oven to 400°F and place a rack in the middle position.
- Heat 1 tablespoon butter in a 10-inch cast iron skillet over medium heat until bubbling. Add bread cubes, toss to coat, and arrange in a single layer. Toast, tossing every minute, until golden brown, about 5 minutes. Transfer croutons to a bowl.
- Reduce heat to low. Add olive oil, onions, and 1/2 teaspoon salt to the skillet. Cook, stirring every 5–10 minutes and scraping browned bits, until onions are soft and deeply caramelized, about 40 minutes.
- Add balsamic vinegar and scrape up any remaining browned bits.
- In a large bowl, whisk together eggs, milk, Dijon mustard, remaining 1 teaspoon salt, and black pepper.
- Stir the remaining tablespoon butter and croutons into the skillet with onions, spreading evenly.
- Pour egg mixture over onions and bread, tilting the pan to distribute evenly. Top with Gruyère cheese.
- Cook on the stovetop for 2–4 minutes until edges begin to set.
- Transfer skillet to oven and bake 8–10 minutes, or until eggs are set. Check doneness by cutting a small slit in the center.
- Cool in skillet for 5 minutes, top with fresh chives, slice into wedges, and serve warm.
Notes
- Caramelize onions slowly for the richest flavor—don’t rush the process.
- Substitute Gruyère with cheddar, Swiss, or mozzarella for variation.
- Use gluten-free bread for a gluten-free option.
- Can be served warm or at room temperature.
- Leftovers store well for up to 3 days in the fridge.
- Prep Time: 15 minutes
- Cook Time: 55 minutes
- Category: Breakfast, Brunch, Main Dish
- Method: Baking, Stovetop
- Cuisine: French
- Diet: Vegetarian
Nutrition
- Serving Size: 1 slice
- Calories: 250
- Sugar: 5g
- Sodium: 620mg
- Fat: 15g
- Saturated Fat: 7g
- Unsaturated Fat: 6g
- Trans Fat: 0g
- Carbohydrates: 18g
- Fiber: 2g
- Protein: 12g
- Cholesterol: 230mg
