Preheat oven to 400°F (200°C). Cook pasta according to package directions. Grease 9" x 13" (3L) baking dish; set aside.
Meanwhile, cook bacon in large high-sided skillet or saucepan set over medium heat for 7 to 8 minutes or until golden and crisp around edges, and fat is rendered. Transfer to paper towel-lined plate to drain; set aside.
Remove all but 1 Tbsp. bacon fat from skillet; cook butter, onion and garlic for about 3 minutes or until softened. Sprinkle in flour; cook, stirring, for 2 to 3 minutes or until combined.
Slowly whisk in milk and cream; bring to boil, whisking constantly. Reduce heat to medium-low; cook, stirring constantly, for about 10 minutes or until thickened. Stir in mustard, nutmeg, salt and pepper. Remove from heat.
Stir in 2 cups of the cheddar and 1/4 cup parmesan until cheese melts. Stir in pasta and two-thirds of the bacon.
Transfer to prepared baking dish. Sprinkle with remaining cheddar, parmesan, and bacon. Bake for about 20 minutes or until golden and bubbly. Let stand for 10 minutes before serving. Sprinkle with chives.
TIP: You can substitute any short-cut pasta, such as penne, orecchiette, shells, or classic macaroni for the fusilli pasta.