Ingredients

• 6  medium potatoes (2 lb.), peeled, if desired, and thinly sliced (about 6 cups)
• 1/2  cup chopped onion (1 medium)
• 2  large garlic cloves, minced
• 2  Tbsp. olive oil or cooking oil
• 1/4  cup all-purpose flour
• 1  tsp. salt
• 1/4  tsp. ground black pepper
• 3  cups milk
• 1  cup shredded Parmesan (4 oz.)

Directions

Preheat oven to 350 degrees F. Cook potatoes in lightly salted boiling water for 5 minutes. Drain; set aside.

For sauce, in a saucepan cook onion and garlic in hot oil over medium heat until tender. Stir in flour, salt, and pepper. Add milk all at once. Cook and stir until thickened and bubbly. Remove from heat.

In a greased 2-1/2- to 3-quart au gratin or rectangular baking dish or two 1-1/2-quart au gratin dishes layer half the potatoes. Pour half the sauce over potatoes. Sprinkle with 1/2 cup cheese. Repeat with remaining potatoes and sauce. Cover and refrigerate remaining cheese.

Bake, covered, for 35 minutes for large dish, and 20 minutes for small dishes. Uncover; top with remaining cheese. Bake 35 minutes more or until potatoes are tender and top is golden. Let stand 10 minutes before serving.

Granny Smith and Smoked Cheddar Gratin: Cut 2 large cored Granny Smith apples in thin wedges. Add 2 teaspoons snipped fresh thyme to sauce. Place half the apples over each layer of potatoes then pour over sauce. Substitute smoked cheddar or Gouda for Parmesan cheese.

Seafood and Goat Cheese Gratin: Add 1 tablespoon snipped fresh tarragon to sauce. Layer 8 ounces smoked salmon or whitefish, skin removed and salmon flaked, on first layer of potatoes before adding sauce. Substitute goat cheese for Parmesan cheese. Served with a green vegetable, this gratin becomes a main dish.

Mushroom and Blue Cheese Gratin: Cook 3 cups sliced cremini mushrooms with the onion and garlic. Add 4 teaspoons snipped fresh sage to sauce. Substitute crumbled blue cheese for Parmesan cheese.

Test kitchen Tip: Gratins can be assembled and refrigerated, covered, up to 24 hours. Bake, covered, in a 350 degree F oven for 50 minutes for a large dish or 40 minutes for small dishes. Uncover; top with remaining cheese. Bake 40 to 50 minutes more or until potatoes are tender.