PLACE cauliflower, broccoli and water in large saucepan. Bring to a boil. Reduce heat to low; cover. Cook for 12 to 15 minutes or until vegetables are crisp-tender. Drain; remove vegetables from saucepan.
MELT margarine in same saucepan over medium heat. Stir in flour. Add evaporated milk; cook, stirring constantly, until mixture comes to a boil and is thickened. Add cheese; stir until melted. Stir in mushrooms; remove from heat. Stir in cauliflower and broccoli.