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In a sauce pan on medium heat melt butter. Add onions and cook until soft.
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Mix flour into butter and onions and cook for 1-2 minutes, until flour is thoroughly combined.
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Season with salt and pepper.
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Add milk and bring to a boil, then simmer for 3 minutes or until sauce thickens.
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Mix in lemon juice, lemon zest and dill.
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Top fish with warm sauce. Optional, add additional fresh dill.