Put the potatoes and 2 tsp of salt in a large saucepan and generously cover with cold water.

Bring to a boil, then reduce heat, and simmer, partially covered, until potatoes are tender, 15 to 20 minutes.

While potatoes are boiling, heat 200ml milk and the 3 tbsp butter in a small saucepan until butter is melted and milk is hot but not boiling. Remove from heat and keep warm, covered.

Drain the potatoes well in a sieve or colander and return to hot saucepan.

Add the milk mixture, Fontina or Gruyère, and black pepper, and mash with a potato masher or fork to desired consistency.

Season with salt and keep warm, covered.

Bring a pot of well-salted water to a boil, then add the cauliflower and garlic and simmer until cauliflower is very tender, 13 to 15 minutes.

Drain cauliflower in a colander and pulse with remaining 100ml milk, 50g parmesan cheese, and the remaining 3 tbsp butter in a food processor until it’s a chunky purée.

Stir together mashed potatoes and cauliflower mixture in a large bowl and season with salt and pepper.

Transfer to a buttered baking dish (not glass).

Heat oven to 425°F with rack in middle. Melt the remaining 2 tbsp butter and gently brush over potato-cauliflower mixture, then sprinkle evenly with remaining 50g parmesan cheese (to taste).

Bake until potato mixture bubbles around edges, about 20 minutes.

Turn on broiler and broil 6 to 8 inches from heat until topping is browned in spots, 1 to 2 minutes.