1. Grease a 9×13-inch baking dish with butter or nonstick spray.

2. Cut the bread into bite-size cubes. If the bread is fresh, toast it at 300°F for 10 to 12 minutes until slightly dry.

3. Melt the butter in a skillet over medium heat, then cook the diced onion for 3 to 4 minutes until softened.

4. Add half of the bread cubes to the baking dish, then layer with half of the ham, onion, cheddar, and mozzarella. Repeat with the remaining bread, ham, onion, and cheese.

5. In a large bowl, whisk together the eggs, whole milk, heavy cream, Dijon mustard, garlic powder, dried thyme, salt, and black pepper.

6. Pour the egg mixture evenly over the bread layers and gently press the bread down so it absorbs the custard.

7. Cover and refrigerate for at least 1 hour, or overnight for the best texture.

8. Preheat the oven to 350°F and let the dish sit at room temperature while the oven heats.

9. Bake uncovered for 45 to 55 minutes, until puffed, golden, and set in the center.

10. Rest for 10 minutes, sprinkle with fresh parsley, then slice and serve warm.