1. Place cauliflower in a steamer insert or basket. Fill a deep, wide pot with 1" water and bring to a boil. Lower insert into pot, cover, and steam until cauliflower is very tender, about 15 minutes.
2. Meanwhile, combine half-and-half and 4 tablespoons butter in a small saucepan over medium heat and cook until butter is melted; cover and set aside.
3. Transfer steamed cauliflower to a food processor. Add garlic-and-herb cheese and half of the half-and-half mixture and pulse until mostly smooth, about 15 times; transfer to a large bowl.
4. Place potatoes in a large saucepan and fill with enough cold water to cover by 2". Bring to a boil and cook until potatoes are tender, 10 to 12 minutes; drain, transfer to a bowl, and mash with remaining half-and-half mixture until mostly smooth. Stir in cauliflower mixture and season with salt and pepper to taste; transfer to a 2-quart broiler-safe baking dish.
5. Place a rack in center of oven and heat to 425 degrees F. Melt remaining 1 tablespoon butter and brush evenly over casserole. Sprinkle evenly with Parmesan cheese and bake until casserole is hot and bubbling around edges, 15 to 20 minutes. Turn oven to broil and continue to cook until top is golden brown, 3 to 5 more minutes.
Source: Hannaford fresh Magazine, October - November 2018