Combine potatoes and 1 teaspoon salt in a medium saucepan; add water to cover. Cook over medium heat 15 to 20 minutes or until tender; drain.
Combine cooked potatoes, onion, and pimiento; stir gently. Spoon into a greased 1 1/3-quart casserole.
Melt 1/4 cup butter in a small 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 pepper, 1/8 teaspoon salt, and cheese; cook, stirring constantly, until cheese is melted.
Pour cheese sauce over potatoes; sprinkle with cracker crumbs, and dot with 2 teaspoons butter. Bake at 350° for 30 minutes or until lightly browned.