- Preheat oven to 350 degrees F.
- Peel potatoes, if desired, and slice thinly (you should have 6 cups). Place slices in a colander. Rinse with cold water; set aside to drain.
- For sauce, in a medium saucepan cook onion and garlic in hot butter over medium heat until tender but not brown. Stir in flour, salt, and pepper. Add LACTAID® Whole Milk all at once. Cook and stir over medium heat until thickened and bubbly. Remove from heat; stir in snipped sage.
- Grease a 2-quart casserole. Layer half of potatoes in casserole. Cover with half the sauce. Repeat layering with the potatoes and sauce.
- Bake casserole, covered, in a 350 degree F oven for 45 minutes. Uncover; bake 15 to 20 minutes more or until potatoes are just tender. Sprinkle 1/3 cup of toasted walnuts over top, if desired. Let stand 10 minutes before serving. Garnish with fresh sage leaves, if desired.