Cook frozen vegetable mix and frozen cauliflower and broccoli mix according to packet directions. Drain.
Preheat grill on medium-high. Meanwhile, melt butter in a saucepan over medium heat. Stir in flour. Cook, stirring, for 2 minutes or until bubbling. Remove from heat. Gradually stir in milk. Return to heat. Cook, stirring, for 5 minutes or until mixture boils and thickens slightly. Remove from heat. Stir in cheese. Season with salt and pepper.
Combine vegetables in a lightly greased, 9cm-deep, 8 cup-capacity flame-proof dish. Pour over cheese sauce. Grill for 5 minutes or until browned. Serve.
