In a large saucepan, saute onion in butter until tender. Stir in the four, salt, pepper and mustard until blended. Gradually add milk. Bring to a boil; cook and stir for 2 minutes or until thickened and bubbly. Reduce heat to low; stir in cheese just until melted.
Stir in beans until coated. Transfer to a greased 2-qt. baking dish. Sprinkle with almonds. Bake, uncovered, at 350° for 25-30 minutes or until bubbly.