Step 1: Grease a 9x13-inch baking dish.
Step 2: Spread half of the bread cubes evenly in the prepared baking dish.
Step 3: Layer half of the diced ham and half of the shredded Swiss cheese over the bread cubes.
Step 4: Repeat the layering with the remaining bread cubes, ham, and Swiss cheese.
Step 5: In a large bowl, whisk together the eggs, milk, heavy cream, sugar, Dijon mustard, salt, and black pepper until well combined.
Step 6: Pour the egg mixture evenly over the bread, ham, and cheese in the baking dish. Gently press down on the bread to ensure it absorbs the liquid.
Step 7: Cover the baking dish tightly with plastic wrap and refrigerate for at least 4 hours, or preferably overnight.
Step 8: When ready to bake, preheat your oven to 375°F (190°C). Remove the casserole from the refrigerator and let it sit at room temperature for about 30 minutes. Sprinkle the top with any remaining shredded Swiss cheese if desired.
Step 9: Bake the covered casserole for 30 minutes. Then, remove the foil and bake for an additional 20-25 minutes, or until the casserole is golden brown and a knife inserted into the center comes out clean.
Step 10: Let the casserole rest for 5-10 minutes before slicing and serving. Dust generously with powdered sugar and serve with raspberry or strawberry jam on the side.