01 - Preheat oven to 400°F. Grease a medium baking dish with approximately 2-quart capacity.
02 - Bring a large pot of salted water to a boil. Add cauliflower florets and cook for 5 minutes until just tender. Drain thoroughly and set aside.
03 - In a medium saucepan over medium heat, melt unsalted butter. Whisk in flour and cook for 1 to 2 minutes to form a roux.
04 - Gradually whisk in whole milk and heavy cream, stirring constantly until smooth and thickened, about 4 to 5 minutes. Stir in Dijon mustard, garlic powder, salt, and black pepper.
05 - Remove sauce from heat and fold in 1 cup Gruyère and ¼ cup Parmesan until fully melted and smooth.
06 - Place cauliflower florets evenly in the baking dish. Pour cheese sauce over the florets. Sprinkle remaining Gruyère and Parmesan evenly on top.
07 - In a small bowl, combine breadcrumbs with melted butter and mix until evenly coated. Sprinkle mixture over the casserole.
08 - Bake for 25 to 30 minutes or until the topping is golden brown and sauce is bubbling.
09 - Allow gratin to rest for 5 minutes before garnishing with chopped fresh parsley and serving.