Coat a 9 x 13 baking dish with nonstick cooking spray, set aside.
In a large skillet over medium high heat, saute the sausage until browned throughout, 7 to 8 minutes. Add the butter and onions to the pan, continue to cook until the onions are tender, another 8 to 10 minutes. Remove from the heat.
In a large mixing bowl, break the croissants into large, bite sized pieces. Add the sausage mix and the shredded cheese. Mix together until all the croissant pieces are well coated. Spread in the bottom of the baking dish.
Using the same mixing bowl, whisk together the eggs, milk, mustard, worcestershire, salt and pepper. Pour over the croissant and sausage mixture. Push down any pieces of croissant that aren’t submerged. Cover and refrigerate overnight. This will allow the croissant pieces to completely soak up the egg mixture.
Preheat an oven to 350 F. While the oven preheats, take the casserole out of the fridge to take off the chill.
Bake for 45 to 50 minutes until the center springs back slightly when touched and the top is golden brown. Let sit for 5 to 6 minutes before serving.