1 tablespoon unsalted butter
3/4 cup fresh bread crumbs
2 tablespoons all-purpose flour 1 1/2 teaspoons dried thyme
1 1/2 teaspoons salt
1 teaspoon ground black pepper
3 russet (Idaho) potatoes, peeled and thinly sliced (Set the sliced potatoes aside in a bowl of cold water to cover to keep them from discoloring until ready to use.)
1/2 small sweet onion, sliced thinly, rings separated
1 1/2 cups chicken or vegetable stock
3/4 cup half-and-half
1 tablespoon Dijon-style mustard, optional
Butter a 9-inch-x-13-inch baking dish. Preheat the oven to 400F.
Melt the butter in a small skillet over medium heat. Add the breadcrumbs and cook, stirring, until golden. Set aside. Whisk together the flour, thyme, salt and pepper. Layer a third of the potatoes then a third of the seasoned flour and half of the onions. Repeat, ending with potatoes.
Whisk together the stock, cream and mustard, if using. Pour this evenly, nearly to the top, over the potatoes. Bake for 25 minutes. Remove from the oven. Scatter breadcrumbs over the top. Return to the oven for an hour longer. The top will be brown, the potatoes tender when pierced, and the liquid mostly absorbed but bubbling lightly around the edges of the potatoes.
Let this rest for 10 to 15 minutes for easier cutting.
From: https://marbleheadcurrent.org/2023/02/22/food-101-scalloped-potatoes-an-american-classic/