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Preheat oven to 375 degrees and place rack in center position
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Start water to boil and cook pasta to package instructions
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While water heats, grate cheddar and slice onions
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Place milk in a small pot on medium heat to warm
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In skillet melt butter on med/high. If not using a skillet melt in a medium sized pot
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When butter is melted add a 1/4 cup green onions and sauté for 2 minutes
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Add flour and whisk until smooth about one minute. This is a roux. A roux thickens your cheese sauce
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Slowly pour warm milk into roux with one hand while whisking with the other
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Add 4 cups cheese one handful at a time whisking between additions (reserving one cup of cheese for later)
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Whisk until all cheese is melted and sauce is smooth
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Season with salt and pepper
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When pasta is ready (do not overcook!) add to skillet with cheese sauce and stir until well incorporated. (If baking in a baking dish, pour pasta into baking dish and then pour cheese sauce over)
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Top with remaining cheese and bake for 20-25 minutes or until cheese is crispy
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Garnish with remaining green onions, serve