Melt 3 tablespoons butter in a heavy saucepan over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add milk; cook over medium heat, stirring constantly, until thickened and bubbly. Stir in salt.
Combine white sauce, ham, eggs, mustard, and pepper; spoon mixture into a 1 1/2-quart casserole. Sprinkle with breadcrumbs, and dot with remaining butter. Cover and bake at 400° for 15 minutes. Uncover and bake an additional 15 minutes or until lightly browned.