Ingredients
Method
Cover bottom croissant halves from 9–10 oz. croissants (about 10 small), split, with a slice of 6 oz. thinly sliced Gruyère and a slice of 6 oz. thinly sliced ham, trimming and tucking as desired to fit; close up with top halves. Cut in half on a deep diagonal.
Whisk 5 large eggs, 2 cups heavy cream, 2 Tbsp. Dijon mustard, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, 1 tsp. freshly ground pepper, and ¼ tsp. freshly grated nutmeg in a large bowl until smooth. Pour half of custard into a 1½-qt. baking dish. Arrange filled croissants in dish, keeping cheese and ham in place as much as possible, then evenly pour remaining custard over to saturate croissants. Cover tightly with plastic wrap and chill at least 1 hour.
Place a rack in upper third of oven and preheat to 325°. Uncover and bake casserole until top is very slightly golden brown, 40–50 minutes.
Heat broiler. Broil casserole just until deep golden brown, about 1 minute. Let cool slightly, then scatter thinly sliced chives over to serve. Do Ahead: Unbaked casserole can be assembled 1 day ahead. Keep chilled.