2 sausage links, cut to 1/4 pieces
1 shallot, minced
1 cup heavy cream
1 cup milk
1 clove garlic, minced
Kosher salt and freshly ground pepper
6 medium yukon gold potatoes (about 2 1/4 pounds), peeled and sliced 1/8-inch thick
1 cup fresh peas, blanched (frozen will also work)
2 cups Gruyere, grated
1/4 cup grated Parmesan
Heat oven to 375F. Butter a 1 1/2 to 2-quart baking dish and sprinkle with just a bit of the shallot.
In a medium sauce pan over medium heat, sauté the sausage links to render the fat. Do not make it too crispy though. Remove with a slotted spoon, but leave the grease in the pan (if there is more than a tablespoon of the rendered sausage fat, remove that too). Add the shallot and sauté until tender, then add the garlic until just fragrant. Add the heavy cream and bring to a simmer while stirring. Let simmer until you see the liquid begin to thicken, 2 to 3 minutes.
Layer 1/3 of the sliced potatoes in the baking dish, cover with some Cheddar and a sprinkling of Parmesan; Sprinkle over some of the peas and sausage. Repeat the layering 2 more times.
Pour the hot liquid over the potatoes. Add the remaining cheese, cover with foil, and bake for 1 hour, then remove the foil to bake until the cheese is golden and brown, 15 to 20 minutes. Let rest for 10 minutes before slicing so it can settle.