Fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the fusilli, and return to a boil. Cook the pasta uncovered, stirring occasionally, until the pasta has cooked through, but is still firm to the bite, about 12 minutes. Drain well in a colander set in the sink.
Preheat oven to 400 degrees F (200 degrees C). Lightly grease a casserole dish.
Melt the margarine in a large saucepan over medium heat; cook the onion in the melted margarine until translucent, about 5 minutes. Whisk the flour into the onion mixture; cook 1 minute more. Slowly pour the milk into the mixture while whisking until the milk is entirely incorporated. Add the cheese food, blue cheese, Cheddar cheese, salt, pepper, and mustard; cook and stir continually the cheese has melted and the mixture is thick; fold the pasta into the mixture. Pour the mixture into the prepared casserole dish.
Bake in the preheated oven until the top begins to brown, about 20 minutes.