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1
Shred cheddar cheese in a food processor (yes, it's a lot). Place shredded cheese in refrigerator while making sauce.
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2
Cook pasta in a large pot of boiling, salted water until the al dente stage (about 10 minutes) and drain. Tip: don't overcook. The pasta will bake later on and overcooking at this stage will make it mushy in the final dish.
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3
Preheat oven to 400u00b0F.
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4
Meanwhile, melt butter in a large saucepan over medium heat. Add onions and cook until they are translucent but not brown, about 5 minutes. Stir in flour until the mixture is smooth.
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5
Gradually add milk to the flour/butter mixture, stirring constantly. Cook and stir for 2 minutes. Add mustard, nutmeg, salt and pepper. Reduce heat and cook, stirring constantly until sauce thickens, about 5 more minutes, Turn off heat, and add two very large hands full of the shredded cheddar cheese to the mixture and stir until the cheese has melted into the sauce.
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6
Combine the cooked pasta and the cheese sauce and pour into a greased 4 quart baking dish. Sprinkle 1/2 the Parmesan cheese over the top, then sprinkle the rest of the shredded cheddar, then the rest of the Parmesan cheese.
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7
Bake at 400 F about 20 minutes, or until lightly browned and bubbly.
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8
Note: This will stay hot 30 - 45 minutes after removing from the oven.
