Grease a 9×13-inch baking dish and arrange the croissant pieces evenly across the bottom.
Sprinkle the ham, chives, Gruyère, and cheddar between and over the croissant pieces.
Sprinkle the ham, chives, Gruyère, and cheddar between and over the croissant pieces.
Pour the custard evenly over the dish. Cover and refrigerate for at least 4 hours or overnight.
Bake uncovered at 350°F or 175°C for 40 to 45 minutes, until puffed and set in the center.
Rest for 10 minutes before serving. Cool leftovers completely before preparing them for the freezer.