Method:

1. Preheat oven to 200C. Grease an 8 cup capacity baking dish. Bring a medium saucepan of water to the boil. Add cauliflower and cook for 4 minutes. Add broccoli florets and cook for 2 minutes. Drain and cool.
2. Combine potato salad, cauliflower, broccoli, chives, salt and pepper in a bowl. Spoon into prepared baking dish.
3. Melt butter in a small saucepan over a medium heat. Add flour and whisk until combined. Cook for 2 minutes or until bubbly. Slowly add milk, whisking constantly until well combined. Continue to whisk until mixture comes to the boil. Simmer for 2 minutes or until thickened. Stir in ½ cup grated cheese. Remove from heat. Spoon sauce over vegetables. Sprinkle remaining cheese over sauce. Bake for 30 minutes or until golden and heated through.