Place 1 in. of water in a saucepan; add vegetables. Bring to a boil. Reduce heat; cover and simmer for 5 minutes or until vegetables are crisp-tender. Drain. In a saucepan, saute onion in butter until tender. Stir in the flour, salt and pepper until blended. Gradually whisk in milk. Bring to a boil; cook and stir for 1-2 minutes or until thickened. Remove from the heat; stir in cheese until cheese is melted. Stir in vegetables.
Transfer to a greased 2-1/2 cup baking dish. Bake, uncovered, at 350° for 20-25 minutes or until bubbly and vegetables are tender. Let stand for 3-5 minutes before serving.