Layers of potatoes and leeks are topped with just a bit of cheese in this healthy gratin.
Place oven rack in the upper third of the oven; preheat to 425 degrees F. Coat a 1 1/2- or 2-quart gratin dish or other shallow baking dish with cooking spray.
Combine milk, garlic, bay leaf, nutmeg, salt and pepper in a large heavy-bottomed saucepan; bring to a simmer, stirring, over medium-low heat. Gently slip in potatoes, partially cover and simmer gently over low heat, stirring often, just until potatoes are tender, but not soft, and the milk has thickened, 10 to 15 minutes. (Alternatively, combine milk and seasonings in a 2-quart glass measure or microwave-proof casserole; microwave on High for 6 to 8 minutes, until simmering. Slip in potatoes, cover with lid or vented plastic wrap and microwave on Medium for 15 to 25 minutes, stirring twice, just until tender.)
Meanwhile, heat oil in a nonstick skillet over medium-low heat; add leeks and saute until tender, about 5 minutes.
Transfer half the cooked potatoes with a slotted spoon to the prepared gratin dish, discarding bay leaf. Cover with leek mixture. Spread remaining potatoes over leeks. Add sour cream and mustard to milk remaining in the saucepan and whisk to blend; spread over the gratin. Sprinkle with cheese. Bake until bubbly and golden on top, 20 to 25 minutes. Serve.
2 starch, 1 vegetable, 1 fat