Add the broccoli, cauliflower and diced potato into an oven-proof dish.
Add the milk to a saucepan and warm. In a small bowl add the flour and a little water to make a paste.
Once the milk is simmering, add the flour paste and stir well. Take off the heat and pour over the broccoli, cauliflower and potatoes.
Sprinkle the cheese over the top and put in the oven at 180C for 25-30 minutes until golden.