1) Sauté the chopped onions and garlic in the oil until they're soft and golden.
2) Add the flour to the onions, stirring to combine.
3) Gradually add the milk, stirring until smooth and thickened.
4) Add the mustard, salt, pepper, and nutmeg.
5) Add the pasta sauce, sugar (if desired), and basil. Stir to combine, and remove from the heat.
6) Preheat the oven to 375°F. Lightly grease a large (about 4-quart), deep casserole dish.
7) Pour about 1 1/2 cups (about one quarter) of the pasta sauce into the bottom of the prepared dish. Add 2 cups (one third) of the uncooked pasta, then about 1 cup (one third) of the cheese. Repeat the layers; then add a final layer of sauce, pasta, sauce, and cheese. Sprinkle with Pizza Seasoning, if desired.
8) Bake the casserole for 35 to 45 minutes, until it's bubbling and beginning to brown on top. Remove it from the oven, and let it rest for 15 to 30 minutes before serving.
Yield: 12 to 20 servings, depending on size.