1. Center a rack in oven and heat to 350 degrees. Grease a broiler-safe 8" baking dish and set aside.
2. Bring a pot of salted water to a boil over high heat. Add pasta, return to a boil, then reduce heat to medium-high and cook until pasta is just shy of al dente, about 5 minutes, adding cauliflower during last minute of cooking. Drain and transfer pasta and cauliflower to a large bowl; set aside.
3. Bring milk to a boil in a medium saucepan over medium-high heat. Reduce heat to medium, then whisk in flour, garlic, mustard, 1 teaspoon salt, and 1/2 teaspoon pepper and cook, whisking constantly, until mixture has thickened, 3 to 4 minutes. Remove from heat and whisk in cheddar cheese until smooth. Pour cheese sauce over pasta and cauliflower and stir until thoroughly combined; season with salt and pepper to taste. Transfer mixture to prepared baking dish.
4. Mix breadcrumbs, Parmesan, parsley, and butter in a small bowl until combined. Sprinkle evenly over pasta mixture, then cover with foil and bake until edges are bubbling, about 20 minutes.
5. Remove foil and broil until breadcrumbs are browned, 2 to 3 minutes. Let sit 10 minutes before serving.
Source: Hannaford fresh Magazine, September 2019