In 4-quart saucepan, melt 3 tablespoons of the butter. Cook onion in butter about 4 minutes, stirring occasionally, until softened. Stir in flour and mustard with whisk; cook about 1 minute. Slowly stir in milk. Cook and stir until bubbly and slightly thickened. Stir in cooked cauliflower, ham and chile slices. Spoon into casserole.