Cooking spray
2 garlic cloves, halved
1 small celeriac (celery root), peeled (about 1 1/2 pounds)
2 pounds Yukon gold or red potatoes, peeled and cut into 1/4-inch-thick slices
4 thyme sprigs, divided
3 tablespoons all-purpose flour
1/2 teaspoon salt
1/4 teaspoon black pepper
2 cups 1% low-fat milk
1 cup (4 ounces) grated Gruyère cheese, divided

Preheat oven to 350°.

Rub the sides and the bottom of an 11 x 7-inch baking dish coated with cooking spray with garlic halves. Crush garlic halves; set aside.

Cut celeriac into 1/4-inch-thick slices; cut slices into quarters to measure 2 cups, reserving remaining celeriac for another use. Place 2 cups celeriac, potatoes, and 3 thyme sprigs in a large saucepan, and cover with water; bring to a boil.

Reduce heat, and simmer 10 minutes or until potatoes are tender; drain and discard thyme.

While the potatoes are cooking, combine flour, salt, and pepper in a small saucepan; gradually add milk, stirring with a whisk. Stir in crushed garlic and 1 thyme sprig. Place the pan over medium heat, and cook until thick (about 10 minutes), stirring constantly with a whisk. Remove from heat; stir in 3/4 cup cheese.

Arrange potatoes and celeriac in baking dish; pour sauce over vegetables. Sprinkle with 1/4 cup cheese. Bake at 350° for 30 minutes or until lightly browned. Discard thyme.

Serves 7.