Following package directions, cook rotini in a large kettle of boiling salted water until al dente.
Drain, rinse with cold water, and drain again.
Meanwhile, melt butter in a wide frying pan over medium-high heat.
Remove sausage casings, crumble meat into pan, add onion, and cook until sausage is browned and onion is limp (about 5 minutes).
Pour off drippings and add cooked pasta, chilies, milk, salt and pepper to taste.
Pour half the mixture into a greased 2 1/2 quart casserole; sprinkle with half the cheese.
Add remaining mixture, arrange tomato slices on top, and sprinkle with remaining cheese.
If made ahead, cover and refrigerate until next day.
Bake, uncovered, in a 35 deg F oven for 25 to 35 minutes or until top is slightly browned.
Makes 6 servings.
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