Prehat oven to 400 degreesF. Butter a 1 1/2 quart casserole dish. In a saucepan, cook mcaroni in lightly salted boiling water until al dente. Drain.
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In a separate saucepan, melt the butter over medium heat. Blend in the flour, mustard and pepper until smooth. Stir while cooking for about 3 minutes. Slowly whisk in the milk. Add cheeses, stirring constantly until the sauce is thick and smooth.
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Stir macaroni into the cheese sauce, and transfer to the prepard casserole dish. Sprinkle bread crumbs over the top, if desired. Cover and bake for 20 to 25 minutes, or until sauce is thick and bubbly. If you like it browned on top, remove the cover during te final 7 minutes.
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NOTE: You are going to think this is too "soupy" when you pour it into the casserole. But don''t worry, it thickens up while baking andwill be just perfect.