Sauté sliced onions in a mix of butter and olive oil over medium heat until golden and caramelized, about 20-30 minutes.

In a separate pan, cook diced bacon on medium heat until it’s crispy, about 5-7 minutes. Remove and set aside on paper towels.

In a large mixing bowl, whisk together eggs, whole milk, salt, black pepper, mustard powder, and thyme until well-blended.

Gently add the day-old bread cubes into the egg mixture, ensuring each piece is well soaked. Let it sit for at least 30 minutes.

Lightly grease a springform pan. Begin layering with soaked bread, followed by caramelized onions, crispy bacon, and a generous sprinkle of cheese.

Preheat your oven to 350°F (177°C). Bake for about 60-80 minutes, or until the strata is set and golden brown.

Let the strata cool slightly before slicing into wedges. Garnish with fresh parsley.