Step 1: Grease a 9x13 inch baking dish with melted butter. Cut the crusts off the bread and arrange 6 slices in a single layer at the bottom of the prepared dish. A good chef's knife can help with efficient crust removal.
Step 2: Layer half of the thinly sliced ham, half of the turkey, and half of the Swiss cheese over the bread slices.
Step 3: Place the remaining 6 slices of bread over the cheese layer.
Step 4: Top with the remaining ham, turkey, and Swiss cheese, ensuring an even distribution.
Step 5: In a large bowl, whisk together the eggs, milk, Dijon mustard, salt, black pepper, and cayenne pepper until well combined.
Step 6: Carefully pour the egg mixture evenly over the bread and meat layers in the baking dish. Press down gently 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, to allow the bread to fully soak up the egg mixture.
Step 8: When ready to bake, preheat your oven to 350°F (175°C). Remove the casserole from the refrigerator and let it sit at room temperature for about 15-20 minutes while the oven preheats.
Step 9: Bake the uncovered casserole for 45-55 minutes, or until it's golden brown and puffed, and a knife inserted into the center comes out clean.
Step 10: Let the casserole cool for 5-10 minutes before slicing and serving. Dust generously with powdered sugar and serve with raspberry jam on the side.