In a large skillet, melt 2 tablespoons of butter with 1 tablespoon of olive oil over medium heat. Add the sliced onions and sauté for about 15-20 minutes until they are soft and caramelized, stirring occasionally. Once done, remove from heat and allow the onions to cool while you prepare the other ingredients.

Using the same skillet, increase the heat to medium-high and add diced bacon. Cook for approximately 5-7 minutes, stirring frequently, until the bacon is crispy and golden. Transfer the cooked bacon to a paper towel-lined plate to drain excess fat, and set aside to cool.

In a large mixing bowl, crack 6 eggs and pour in 2 cups of whole milk. Whisk together vigorously until fully combined, ensuring the mixture is smooth. Season with 1 teaspoon of salt, ½ teaspoon of black pepper, and 1 teaspoon of mustard powder if using.

Add the cooled bread cubes from about a half baguette or French bread to the egg mixture. Then, fold in the caramelized onions, crispy bacon, and half of the grated cheese, about 1 cup. Mix thoroughly until everything is well combined, letting it sit for at least 30 minutes to let the bread absorb the flavors.

Preheat your oven to 350°F (177°C) while the mixture rests. Grease a 9x13 inch baking dish and pour in the Savoury Baked French Toast mixture, spreading evenly. Top with the remaining cheese. Bake in the preheated oven for 50-60 minutes, or until the top is puffed and golden.

Once baked, remove the dish from the oven and let it rest for about 5-10 minutes. Optionally, sprinkle chopped parsley over the top before cutting into squares to serve warm.