1. Cook the macaroni to packet directions. Bring a large pot of lightly salted water to a boil. Cook elbow macaroni in the boiling water, stirring occasionally until cooked through but firm to the bite, 8 minutes. Drain.
2. While the macaroni is cooking, melt the butter in a saucepan. Remove from the heat and mix in the flour until smooth. Return to the heat and cook stirring until the mixture is frothy. Mix in the mustard. Remove from the heat and mix in a little milk at a time, stirring after each addition until smooth. Return to the heat and cook until the sauce boils and thickens. Mix in the salt and cheese. (Melt butter in a saucepan over medium heat; stir in flour, salt, and pepper until smooth, about 5 minutes. Slowly pour milk into butter-flour mixture while continuously stirring until mixture is smooth and bubbling, about 5 minutes. Add Cheddar cheese to milk mixture and stir until cheese is melted, 2 to 4 minutes.)
3. Drain pasta well, shaking the macaroni to remove as much water as possible Spoon into a serving dish and grill until lightly golden. (Bake in preheated oven ❲350 degrees F (175 degrees C) ❳ until the top is golden brown, 35 to 40 minutes.)
白酱里加上蘑菇会更香。