2 tbsp butter
1 small onion, minced
4 cups 1/2 & 1/2
4 sprigs fresh thyme
2 bay leaves
2 tsp salt
1/2 tsp ground black pepper
4 lb. russet potatoes, peeled and cut into 1/8-inch-thick slices
1 cup shredded cheese of your choice: Gruyere, Parmesan or Cheddar work well (about 4 ounces)
Heat oven to 350 degrees. Melt the butter in a large pan. Add onion and sauté until it begins to brown, about 4 minutes. Add 1/2 & 1/2, thyme, bay leaves, salt, pepper, and potatoes and bring to simmer. Cover, adjusting heat as necessary to maintain light simmer, and cook until potatoes are almost tender (paring knife can be slipped into and out of center of potato slice with some resistance), about 15 minutes.
Remove and discard thyme sprigs and bay leaves. Transfer potato mixture to 3-quart gratin dish and sprinkle with cheese. Bake until liquid has thickened and is bubbling around sides and top is golden brown, about 20 minutes.
Cool for 5 minutes before serving.
Note: At exactly 1/8 inch, the potatoes held their shape yet remained flexible enough to form tight, cohesive layers. Although it is possible to cut 1/8-inch slices by hand, it is far easier when using a food processor or mandoline. Cut thinner, the potatoes disintegrate when cooked. Cut thicker, they slide apart when served.
2026: For a group of 10 people 2.5# potatoes would be perfect. I made a white sauce with a bit of cheese in it to be sure the top was covered. Make sure the liquid is salted well. The amounts in the recipe are off. The fresh thyme is delicious.