Preheat the oven to 375°F.
Butter a 3-quart casserole.
Put the cooked macaroni into the casserole, pour the cheese sauce over it, and mix gently with a fork.
Sprinkle the grated cheese evenly over the top and spread the crumbs over the cheese.
Bake, uncovered, until the top is golden and the sauce is bubbling, about 30 minutes.