1. Grease a 9×13 inch baking dish with butter or nonstick spray. Cut the brioche or challah into 1 inch cubes and spread half of the bread in an even layer in the prepared dish.

2. Sprinkle half of the chopped ham and turkey over the bread, then top with half of the Swiss and Gruyere cheese. Repeat the layers with the remaining bread, meats, and cheese.

3. In a large mixing bowl, whisk together the eggs, whole milk, half and half, Dijon mustard, maple syrup, vanilla extract, kosher salt, black pepper, and ground nutmeg until the mixture is completely smooth and well combined.

4. Slowly pour the custard evenly over the layered bread, meats, and cheese, making sure to cover all corners of the dish. Gently press the bread down with the back of a spatula so it absorbs the liquid.

5. Dollop the raspberry or strawberry jam over the top of the casserole in small spoonfuls, allowing some of it to sink between the bread cubes.

6. Cover the baking dish tightly with plastic wrap or foil and refrigerate for at least 30 minutes, or up to overnight, to let the bread soak up the custard.

7. When ready to bake, preheat the oven to 350°F (175°C). Uncover the casserole, drizzle the melted butter evenly over the top, and place the dish on the center rack of the oven.

8. Bake for 40 to 45 minutes, or until the casserole is puffed, the top is deep golden brown, and the center is set and no longer overly jiggly. If it browns too quickly, tent loosely with foil for the last 10 minutes.

9. Remove the casserole from the oven and let it rest for 10 to 15 minutes so it can set and slice cleanly. Dust the top with powdered sugar, sprinkle with chopped fresh parsley, slice into squares, and serve warm with extra jam or warm maple syrup if desired.