Tear the croissants into 1-inch pieces and spread them on a baking sheet. Toast them in a 350°F oven for about 5-7 minutes, stirring halfway through, until they're lightly golden and slightly dried out—this prevents them from becoming soggy in the custard. While the croissants toast, grease a 9x13 inch baking dish with butter or cooking spray, then dice the ham into 1/2-inch cubes and shred the Swiss cheese. Once the croissants have cooled slightly, transfer them to the prepared baking dish and scatter the diced ham and shredded cheese evenly throughout.

In a medium bowl, whisk together the eggs, half and half, Dijon mustard, salt, and black pepper until completely smooth and well combined. I like using Dijon mustard here because it adds a subtle savory depth that complements the ham without overpowering the delicate cheese and croissant flavors. Make sure the eggs are well incorporated to create an even custard that will cook uniformly.

Pour the custard mixture from Step 2 evenly over the croissant, ham, and cheese mixture in the baking dish from Step 1. Gently stir with a fork to ensure the custard distributes throughout and all the croissant pieces are moistened. Cover the dish tightly with plastic wrap or aluminum foil and refrigerate for at least 30 minutes—this allows the croissants to absorb the custard and helps the flavors meld together for a more cohesive dish.

Preheat your oven to 350°F and place the chilled casserole (still covered) on the middle rack. Bake for 20 minutes covered to gently cook the eggs and warm the filling without drying it out. Then uncover the dish and bake for an additional 10-15 minutes until the top is golden brown and a knife inserted in the center comes out clean. I find that baking covered first prevents the top from browning too quickly while the center finishes cooking.

Remove the casserole from the oven and let it rest for 5 minutes before serving—this allows the custard to set completely and makes it easier to cut clean portions. Dust the top generously with powdered sugar just before serving for a subtle sweetness that balances the savory ham and cheese. Serve warm directly from the dish or cut into squares for individual portions.