Butter the bread slices on one side and set aside.
In a large skillet, melt the butter and saute the mushroom and onions over medium heat until lightly browned and tender.
In a greased 11x17 inch shallow casserole dish, layer half the bread, mushroom mixture, kielbasa and cheese. Repeat the layers, ending with cheese.
In a medium mixing bowl, whisk together eggs, milk, mustards, nutmeg and a sprinkle of salt and pepper. Pour over the casserole; cover and refrigerate.
THE MORNING OF
Preheat oven to 350 degrees F. Let casserole sit at room temperature for 15 minutes.
When ready to bake, sprinkle parsley over the top of the casserole and bake for one hour, or until bubbly.
Notes
If you don't wish to use kielbasa, you can substitute a mild Italian sausage. Cook and cut into bite sized pieces. The kielbasa is already pre-cooked, so you don't have to cook it before adding it to the casserole.
If you don't have an 11x17 inch casserole dish, improvise by using a couple of smaller sized dishes.