Preheat the griddle. Brush one side of each piece of broiche with butter. Spread the other half with mustard. On four slices of bread, mustard side up, lay a piece of cheese. Place two slices of sausage on top of the cheese. Lay a piece of cheese over the sausage. Place two slices of sausage on top of the cheese. Lay a piece of cheese over the sausage. Top each pile with the remaining bread, mustard side down. In a mixing bowl, whisk the eggs and cream together. Season with salt and pepper. Dip sandwiches in the egg mixture and place on a hot griddle and cook for 2 to 3 minutes on each side, or until golden brown and the cheese has melted.
Slice the sandwiches in half and serve with the fried onions.
Combine all ingredients thoroughly and store in an airtight jar or container.
Recipe from "New New Orleans Cooking", by Emeril Lagasse and Jessie Tirsch. Published by William and Morrow, 1993.