Start by slicing one large onion and dicing 6 strips of bacon into small pieces. Cut about 6 cups of day-old bread into cubes (about 1-inch pieces).

In a large skillet over medium heat, melt 2 tablespoons of butter with 2 tablespoons of olive oil. Add the sliced onions and sprinkle with a pinch of salt. Sauté for about 15-20 minutes, until golden and soft.

Add the diced bacon to the same skillet and cook over medium heat until crispy, about 6-8 minutes. Transfer to a paper towel-lined plate.

In a large mixing bowl, whisk together 6 large eggs, 2 cups of milk, 1 teaspoon of salt, ½ teaspoon of black pepper, and 1 teaspoon of mustard powder if using. Include 1 teaspoon of fresh thyme.

Fold the cubed bread into the custard mixture until coated, then fold in the caramelized onions, bacon, and 2 cups of shredded cheddar cheese.

Grease a 9x13-inch baking dish or springform pan with butter. Pour the mixture into the prepared dish and press down gently with a spatula.

Cover the dish with plastic wrap and let it sit at room temperature for at least 30 minutes, or refrigerate for up to 24 hours.

Preheat your oven to 350°F (177°C). Remove the cover and bake uncovered for about 45-60 minutes, until the top is golden brown.

Allow to cool for about 10 minutes before slicing. Garnish with fresh parsley or herbs and serve warm.