Add the potatoes to a large pan and cover with water. Bring to a boil, add the 2 tablespoons salt and simmer for 15 minutes until tender when a fork is inserted. Drain and add to a large bowl. Transfer back to the hot dry pan (to allow the potatoes to dry out).

Preheat oven to 375°F/190°C.

Butter the inside of a 9 x 13-inch/23 x 33 cm/4 quart casserole dish.

Run the potatoes through a ricer or food mill or mash really well to get the smoothest mash possible.

Add the cream, butter, cheeses, rosemary and pepper. Stir until combined and the cheese is melted. Taste and add salt if needed (I did not since I added it to the boiling water as well as the cheeses have salt).

Pour the mash into the casserole dish and spread evenly. If making ahead, cover at this stage without the breadcrumbs and refrigerate. Bring to room temperature before adding the breadcrumbs then baking.

To freeze ahead, cover the casserole well. The day before you’re ready to bake, take it out of the freezer and thaw in the fridge overnight. Make sure you bring it to room temperature before baking (this is important if it is in the baking dish so it is not too cold for a hot oven). This is a good time to prepare the panko topping, then bake.

Mix the breadcrumbs and Parmesan in a bowl. Sprinkle an even layer of breadcrumbs and Parmesan cheese on top of the mashed potato.

Bake for 30 minutes until the top is golden.