Two potato gratin with Gruyere, garlic and cream

I wouldn't recommend you eat this dish more than once a month even. It's unctuous in the most delicious and naughty way. As Christmas comes but once a year, this is perfect. Full of cream, nutty cheese, garlic and thyme, it's a riff on the classic potato Dauphinoise, with a North American twist of adding sweet potatoes. Feel free to make it with all the regular kind. Yukon gold are particularly good here. 

Recipe : Two potato gratin

2 medium sized sweet potatoes

3 medium sized potatoes

2 cloves garlic, chopped finely

6 sprigs thyme, leaves picked 

salt and freshly ground black pepper

1/2 cup grated Gruyere

1/2 cup cream

1 tbsp butter

Lightly butter a baking dish, preferably glass so you can see all the pretty layers of this dish and pre heat the oven to 375oF.

Peel all the potatoes and slice as thinly as possible. You may find it easier to cut off a little piece at one end so you can stand the potato on a surface without it wobbling around, which is safer and easier for you. 

Start layering the potatoes into the dish in a single layer. You can use a mixture of both in one layer or just one type, whatever you like. 

Once your first layer is down, season well with salt and pepper, a sprinkling of thyme leaves, a few pieces of garlic and then start on the next layer. Continue in this way until you reach the second to last layer. Season as before but now also add the Gruyere, spreading it evenly over the potatoes. Now finish with a final potato layer, season again with salt, pepper and thyme and then pour the cream over the top where it should flow down like lava and find it's way through the whole dish. 

Dot the butter over the top and bake in the oven for between 1 to 1.5 hours by which time the cream should be mostly incorporated and evaporated, the potatoes soft and the top golden brown. When you first take it out of the oven you may find that there is still quite a bit of liquid left. I covered and refrigerated it overnight, where it became quite solid. The next day I found some oil from the butter and cream had risen to the top, so I mopped it up slightly with paper towels. When I then reheated, it stayed solid and I was able to successfully cut it into firm wedges.