Heat oven to 400. Cook the pasta for half the time recommended on package directions; drain.
Meanwhile, in a large bowl, whisk together the mustard, 1 cup of the sour cream, and 1/2 tsp each salt and pepper. Fold in the onion and 1 cup of the cheddar.
Add the pasta to the bowl and toss to coat. Transfer the pasta mixture to a shallow 3-qt. Or 9x13 baking dish and bake until beginning to brown, 15 to 20 minutes.
Remove the baking dish from the oven and fold in the remaining cup of cheddar and 1/4 cup sour cream. Return to the oven and bake until golden brown, 15 to 20 minutes more. Serve with freshly cracked pepper.
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