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If not making ahead of time, preheat oven to 350 degrees.
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Grease a 1 1/2 qt baking dish.
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Peel and slice carrots. Cover with salted water and bring to a boil, cook for 15 minutes or until tender. Drain and place in baking dish.
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Meanwhile, saute onion in butter over medium heat until translucent.
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Add flour to onions, stirring to incorporate. Then slowly add milk, stirring as you go. Add salt, sugar and dry mustard and stir again.
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Pour white sauce over carrots. Combine bread crumbs and melted butter, sprinkle over top of casserole.
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If making ahead, cover casserole and put in fridge until needed. Remove 1/2 hour before coooking and preheat oven to 350 degrees.
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Bake for 25-30 minutes or until heated through.