1/4 cup butter
3 cloves garlic minced
1/4 cup cornstarch
1 teaspoon chopped fresh thyme
3/4 teaspoons salt
1/2 teaspoon pepper
2 1/2 cups milk or milk alternative
6 Yukon Gold potatoes
1 small onion sliced
In saucepan, melt butter over medium heat; cook garlic, stirring, for 1 minute. Add cornstarch, thyme, salt and pepper; cook, stirring, for 1 minute.
Gradually whisk in milk; cook, whisking constantly, until boiling and thickened, about 8 minutes.
Peel and thinly slice potatoes. Layer one-third in greased 8-inch (2 L) square baking dish or casserole dish; spread half of the onion over top. Repeat layers. Arrange remaining potatoes over top. Pour sauce over top, using back of knife to ease sauce between layers.
Cover and bake in 350°F (180°C) oven for 1 hour. Uncover and bake until lightly browned and potatoes are tender, about 30 minutes. Let stand for 5 minutes before serving.