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.