Preheat the oven to 350°F (175°C).

Cook the elbow macaroni according to package directions, aiming for al dente. Drain and set aside.

Melt 1/3 cup of butter in a Dutch oven over medium heat and sauté the onion until tender.

Whisk in the all-purpose flour, ground mustard, salt, and pepper. Gradually add the milk, stirring until thickened.

Stir in the shredded sharp cheddar cheese until fully melted into the sauce.

Combine the pasta with the cheese sauce, mixing until coated.

Transfer to a greased 13x9-inch baking dish.

Melt the remaining butter and toss with breadcrumbs. Sprinkle over the macaroni mixture.

Bake uncovered for 30-35 minutes until bubbly and golden brown.