Cut the broccoli and cauliflower florets into even sized pieces. Bring a pan of salted water to the boil. Boil the veg for around 10mins. Drain and allow to stand in the colander.
Preheat the oven to 200°c/gas 6.
Dry fry the bacon until crisp, remove and set aside.
In a saucepan, over a low to medium heat melt the butter and using a balloon whisk, stir in the flour. Once all combined, gradually add the milk, stirring all the time with the whisk until it boils, simmer until the sauce has thickened.
Add two thirds of the cheese and stir until melted and then season.
Place the vegetables into a shallow dish, pour the cheese sauce all over and sprinkle the remaining cheese, top with bacon and place in the oven for 15mins or until brown and bubbling. Serve immediately.