Thinly-sliced potatoes layered with cheese and baked in a cream sauce.
Begin heating the cream in a small pot over low heat.
Peel the garlic and slice each clove in half. Peel the shallots and cut into quarters.
Add the garlic, shallots, salt, white pepper, thyme, and rosemary to the cream and bring to a simmer on low heat. Continue with the remaining steps while the cream sauce is heating and simmering, but remember to stir it occasionally.
Grate the cheese if it isn’t already grated. Preheat the oven to 325º F (165º C).
Use the butter to grease the bottom and sides of a 9x13 baking dish.
Scrub the potatoes, then slice them thinly (2mm or less) on a mandolin or using a food processor - no need to peel. Check on the cream sauce - when the shallots and garlic are soft, remove it from heat and let it cool a bit. When it’s a safe temperature, transfer to a blender or use an immersion blender to blend until smooth.
Line the bottom of the dish with a layer of shingled, overlapping potatoes, then grind some pepper on top and sprinkle with 50 g (1/2 cup) of the cheddar. Add additional layers of potatoes, pepper, and cheddar until you add your fifth or sixth potato layer, which should be about half an inch from the top of your dish. There will be potatoes on top when you're done.
Pour the cream sauce over the potatoes, being sure to coat all of the top layer of potatoes. Bake for 60-75 minutes until the potatoes are tender.
OPTIONAL: Sprinkle another 100 g (1 cup) cheddar on top and broil until the cheese starts to get bubbly and brown. If you only have five layers of potatoes, you should have about 50 g (1/2 cup) remaining. The downside of adding cheese is that the dish gets a bit greasy. The plus side is more cheese.