Preheat oven to 350 degrees.
Spray a 13 x 9 baking dish with nonstick cooking spray.Slice each croissant in half. Place the bottom halves in prepared baking dish. Place a slice of ham and half a slice of cheddar on top of each croissant bottom. Place tops of croissants over cheese.
In a small bowl, whisk melted butter, minced onions, mustard, and poppy seeds until combined. Pour over croissants, using a pastry brush to make sure the croissants are fully coated with the butter mixture.
Cover with foil. Bake 15 minutes, remove foil, and bake an additional 3 minutes or until browned and cheese is melted. Serve warm.