Step 1: Prepare Your Ingredients. Dice your cooked ham into small, even pieces. Shred your chosen cheese. If using day-old bread, cube it into 1-inch pieces. For fresh bread, you might want to lightly toast it in the oven for 10-15 minutes to dry it out slightly. This prevents sogginess.
Step 2: Grease Your Baking Dish. Lightly grease a 9×13-inch baking dish with butter or cooking spray. This crucial step prevents sticking and makes cleanup much easier.
Step 3: Whisk the Egg Mixture. In a large bowl, vigorously whisk together the large eggs, milk (or half-and-half/cream), Dijon mustard (for a subtle tang), salt, black pepper, onion powder, and garlic powder. Whisk until the mixture is completely uniform and slightly frothy. This ensures the seasonings are well distributed and helps create a fluffy texture.
Step 4: Assemble the Casserole. Spread half of the bread cubes evenly over the bottom of your prepared baking dish. Sprinkle half of the diced ham and half of the shredded cheese over the bread. Repeat with the remaining bread, ham, and cheese, creating layers.
Step 5: Pour and Soak. Carefully pour the egg mixture evenly over the layered ingredients in the baking dish. Gently press down on the bread and fillings with a spoon or spatula to ensure everything is submerged and gets a good soak. This is vital for a moist, flavorful casserole.
Step 6: Chill and Marinate. Cover the baking dish tightly with plastic wrap and refrigerate for at least 30 minutes, or ideally, overnight (8-12 hours). This allows the bread to fully absorb the egg custard, resulting in a cohesive and flavorful casserole with a perfect texture.
Step 7: Preheat and Bake (Covered). 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. Cover the baking dish tightly with aluminum foil. Bake for 30 minutes.
Step 8: Bake (Uncovered) and Finish. After 30 minutes, remove the foil. Continue baking for another 20-30 minutes, or until the casserole is puffed, golden brown on top, and a knife inserted into the center comes out clean. The internal temperature should reach 160°F (71°C).
Step 9: Rest and Serve. Once baked, remove the casserole from the oven and let it rest for 5-10 minutes before slicing and serving. This resting period allows the custard to set further, preventing it from falling apart when cut. Serve warm and enjoy!