Cook broccoli and cauliflower in saucepan in water until just crispy tender. Set aside. Make white sauce by melting butter in another pan over low heat. Blend in flour and salt. Add milk all at once. Cook quickly, stirring constantly until mixture thickens and bubbles. Add cheese. Stir until melted and smooth. Combine vegetables and sauce in slow cooker, Mix well. Cook on low 1 1/2 hours.