- Preheat oven to 350 degrees F.
- Lightly coat the inside of a 2-quart baking dish with cooking spray, or the insides of six 1 ½ – cup baking dishes
- In a medium saucepan, melt the butter over medium-low heat. Stir in the onions and sauté until translucent. Stir in the mustard powder and flour until well blended.
- Gradually pour in the milk, bring to a simmer, stirring constantly, and cook for 3 minutes, using a whisk if there are lumps. Season with salt and pepper. Blend in the fontina, cheddar, Parmesan cheese, Canadian bacon and chives. Stir until smooth and hot.
- Remove from heat. Fold in the ziti.
- Pour the pasta into the prepared baking dish and sprinkle with crushed cheese crackers. Bake 30-40 minutes for a large casserole, 25-30 minutes for individual dishes, until very hot and beginning to brown on the edges. Serve hot.