Coat a 9x13 inch baking dish with cooking spray. Layer bread, kielbasa, onions, and 1 1/2 cups cheese in pan. In bowl combine eggs, milk, dry mustard, salt, and pepper. Mix well and pour over mixture in the pan, making sure to coat all pieces of bread. Sprinkle with remaining cheese and refrigerate overnight.
The next morning, preheat oven to 375 degrees F (190 degrees C). Combine half and half with nutmeg; pour evenly over the casserole.
Bake in preheated oven for 45 to 55 minutes, until golden and puffed. Allow to cool 15 minutes before serving.
