Arrange a steamer rack in a pot over boiling water. Place cauliflower on rack, cover, and steam for 5 minutes. Mix in the green beans and continue steaming for 2 to 3 minutes. Take off of heat.
In the meantime, melt margarine in a large pot. Mix in the flour and milk. Stir until there are no lumps and cook until milk bubbles. Mix in the cheese and mustard. Pour over hot vegetables and combine.