Place potatoes in large saucepan, cover with cold water. Add 1/2 teaspoon salt and bring to a boil. Reduce heat to low and cook 20 to 25 minutes or until potatoes are tender, drain; reserve. Melt 2 tablespoons butter in medium saucepan. Add flour, salt, and pepper; cook until bubbly. Add milk; stir well; cook, stirring constantly, until thickened. Combine potatoes, hard-boiled eggs, onion, salt, and pepper in lightly greased 1 1/2 quart casserole. Add white sauce; blend lightly with spatula. Melt remaining 1 tablespoon butter. Add breadcrumbs; mix well. Sprinkle buttered crumbs over casserole. Bake at 350F for 30 minutes. Serve hot!