Mashed Potato and Cauliflower Gratin

Ingredients

• 0.3 teaspoon pepper black freshly ground
• 2 pound cauliflower cut into 1 1/2-inch pieces, including stems and core
• 1 cup coarsely fontina italian packed grated
• 3  garlic cloves peeled smashed
• 0.8 cup parmigiano-reggiano divided grated
• 2 pounds russet potatoes peeled cut into 1-inch pieces (baking)
• 8 servings salt 
• 8 tablespoons butter unsalted divided
• 1.3 cups milk whole divided

Equipment

• food processor
• bowl
• sauce pan
• oven
• pot
• sieve
• baking pan
• potato masher
• broiler
• colander

Directions

1. Put potatoes and 2 teaspoons salt in a large heavy saucepan and generously cover with cold water (about 2 1/2 quarts). Bring to a boil, then reduce heat, and simmer, partially covered, until potatoes are tender, 15 to 20 minutes.
2. While potatoes are boiling, heat 3/4 cup milk and 3 tablespoons butter in a small saucepan until butter is melted and milk is hot but not boiling. Keep warm off heat, covered.
3. Drain potatoes well in a sieve or colander and return to hot saucepan.
4. Add milk mixture, Fontina, and pepper, and mash with a potato masher or fork to desired consistency. Season with salt and keep warm, covered.
5. Bring a pot of well-salted water to a boil, then add cauliflower and garlic and simmer until cauliflower is very tender, 13 to 15 minutes.
6. Drain cauliflower in a colander and pulse with remaining 1/2 cup milk, 1/2 cup Parmigiano-Reggiano, and 3 tablespoons butter in a food processor until it's a chunky purée.
7. Stir together mashed potatoes and cauliflower mixture in a large bowl and season with salt and pepper.
8. Transfer to a buttered 3-quart flameproof shallow baking dish (not glass).
9. Heat oven to 425°F with rack in middle. Melt remaining 2 tablespoons butter and gently brush over potato-cauliflower mixture, then sprinkle evenly with remaining 1/4 to 1/2 cup Parmigiano-Reggiano (to taste).
10. Bake until potato mixture bubbles around edges, about 20 minutes.
11. Turn on broiler and broil 6 to 8 inches from heat until topping is browned in spots, 1 to 2 minutes.
12. •Potato and cauliflower mixture, without final melted butter and cheese topping, can be made 2 days ahead and chilled, covered. Bring to room temperature, then bake and broil with topping.•Recipe can be halved and baked in a 2-quart flameproof shallow baking dish.