- Preheat oven to 350°F (175°C).
- Cook macaroni according to package directions; drain.
- In a large bowl, combine cheese, macaroni, milk, dry mustard, onion, soup, and salt.
- Add salmon and frozen peas.
- Place in a greased 9- by 13-inch pan.
- Cover mixture with bread crumbs; drizzle with melted butter.
- Bake for 45 minutes.