Melt butter, saute onions until soft; add garlic powder, salt and pepper.
Stir in flour then add milk, heat until thickened.
Add 1 cup of both cheeses; stir.
Mix sauce, sausage and pasta.
Pour into 13x9" greased casserole dish.
Layer tomatoes on top.
Top with remaining cheddar.
Bake 350 for 30 minutes.
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