Spray an 8x8-inch baking pan with cooking spray.

Cut croissants into 1-inch cubes (about 4 cups, loosely packed) and spread out on the bottom of the prepared pan. Sprinkle evenly with cheese and ham.

Combine eggs, milk, sour cream, Dijon mustard, salt, and pepper in a bowl. Beat the mixture until well blended using a wire whisk or an egg beater.

Pour egg mixture evenly over the top of the croissants, cheese, and ham.

Cover tightly with aluminum foil and place in the refrigerator for 8 hours.

When ready to bake, set the dish out of the refrigerator on the counter. Preheat the oven to 350°F (175°C).

Uncover casserole, place in the center of the preheated oven, and bake until the casserole is set, and a knife inserted into the middle of the casserole comes out clean, 45 to 55 minutes.