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In a small bowl, beat egg.
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In a large pot, melt butter and sprinkle in flour. Whisk together over medium-low heat. Cook mixture for five minutes, whisking constantly. Do not let it burn.
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Pour in milk, add mustard, and whisk until smooth. Cook for five minutes until very thick. Reduce heat to low.
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Take 1/4 cup of the sauce and slowly pour it into beaten egg, whisking constantly to avoid cooking eggs. Whisk together till smooth.
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Pour egg mixture into sauce, whisking constantly. Stir until smooth.
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Add in cheese and stir to melt.
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Add salt and pepper. Taste sauce and add more salt and seasoned salt as needed!
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Pour in drained, cooked samp and stir to combine.
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Serve immediately (very creamy) or pour into a buttered baking dish, top with extra cheese, and bake for 20 to 25 minutes or until bubbly and golden on top.