Step 1

Heat broiler. In a large straight-sided, oven-safe skillet, melt butter. Remove 1 tablespoon butter and combine with breadcrumbs and Parmesan.

Step 2

Add onion to skillet and cook until softened. Whisk in flour and cook, stirring, 1 minute. Very gradually whisk in milk and bring to a simmer. Add macaroni and cook, stirring constantly and scraping bottom of pan, until tender, about 6 minutes. Remove from heat, stir in remaining cheese and the mustard, and season with salt and pepper. Top with breadcrumb mixture and broil until golden brown, 1 to 2 minutes.