1.Preheat oven to 180°C. Lightly grease a 20cm x 30cm casserole dish.
2.Place cauliflower and broccoli in a large saucepan. Cover with water. Bring to boil on high heat. Reduce heat to low and simmer, covered, for 6-8 minutes, or until tender. Drain well. Arrange in prepared dish.
3.Melt butter in a saucepan on medium heat. Stir in flour and cook for 1 minute. Remove from heat and gradually mix in warmed milk, until smooth. Return to medium heat. Stir until sauce boils and thickens. Simmer for 3 minutes.
4.Pour sauce over vegetables. Sprinkle with combined cheeses and breadcrumbs.
5.Bake for 20-30 minutes, or until gratin is bubbling and golden. Serve with salad.
If desired, add a washed and trimmed leek to this mixture.