Lightly coat a 9×13-inch baking dish with cooking spray. Add cubed potato bread, diced ham, and shredded cheddar in an even layer.
Make Custard
In a large bowl, whisk together eggs, half-and-half, salt, pepper, dried mustard, and garlic powder until glossy.
Pour the custard over the bread mixture, pressing gently so all bread is moistened.
Cover with foil and refrigerate for 8 to 24 hours to let flavors meld and bread absorb the custard.
Preheat oven to 350°F and let the casserole sit at room temperature for 30 minutes before baking.
Bake uncovered for 45 minutes until the top is golden and bubbling.
Notes
Pressing the bread gently into the custard before covering avoids dry pockets.Leftovers can be frozen in individual portions for up to 1 month; thaw overnight before reheating.