Split croissants. Place 2 slices ham in each croissant. Place in baking dish. Melt butter over medium heat in saucepan, then stir flour until smooth. Cook for 30 to 45 seconds or until mixture is bubbly, stirring frequently.
Add milk. Cook until mixture begins to boil and thicken, stirring constantly with whisk. Season with salt, pepper and nutmeg; reduce heat. Simmer for 3 to 4 minutes. Add cheese and onion. Bring to a boil, whisking constantly. Pour over and around croissants.
Bake at 325° for 30 to 40 minutes or until bubbly. These would also be great with a little broccoli or spinach sandwiched with the ham.