In a large skillet, melt 2 tablespoons of butter with 1 tablespoon of olive oil over medium heat. Add 2 sliced medium onions and cook, stirring occasionally for about 15–20 minutes, until they are golden brown and soft.

In the same or a separate skillet, cook 6 ounces of diced bacon over medium heat until it's crispy, about 5–7 minutes. Stir occasionally. Transfer the bacon to a paper towel-lined plate to cool.

In a large mixing bowl, whisk together 6 large eggs, 2 cups of whole milk, 1 teaspoon of salt, 1/2 teaspoon of black pepper, and 1 teaspoon of mustard powder until well-combined.

Add 1 large cubed baguette, the caramelized onions, the cooked bacon, and 1 cup of grated cheese to the egg mixture. Gently stir until all the bread is well coated.

Preheat your oven to 350°F (175°C). Grease a baking pan. Pour the mixture into the pan, spreading it out evenly. Bake for 50–60 minutes, until the top is golden brown.

Once baked, let it cool for a few minutes. Slice into wedges, serve warm, and garnish with fresh herbs like parsley or thyme.