If desired, peel potatoes. Thinly slice potatoes (you should have 6 cups). Place slices in a colander. Rinse with cool water; set aside to drain.
Preheat oven to 350 degrees F. For sauce, in a medium saucepan, heat walnut oil over medium heat. Add onion and garlic; cook until tender but not brown. Stir in flour, salt, and pepper. Add milk all at once. Cook and stir over medium heat until thickened and bubbly. Remove from heat; stir in snipped sage.
In a greased 2-quart casserole with a lid, layer half of the potatoes. Cover with half of the sauce. Sprinkle with half the cheese. Repeat layering with the potatoes and sauce. Reserve remaining cheese.
Cover casserole. Bake for 40 minutes. Uncover and bake about 25 minutes more or just until potatoes are tender. Sprinkle remaining cheese and nuts over top. Bake, uncovered, for 5 minutes. Remove from oven; let stand for 10 minutes. If desired, garnish with sage leaves.