1. Steam the broccoli and cauliflower al dente. Refresh and drain well.
2. Melt the butter and the flour and cook for 2 minutes, gradually add the milk, hot, stir to form a thick, smooth sauce. Season and cook for a further 10 minutes, stir in 100g of the cheese and cook until fully melted.
3. Cook the pasta shells in boiling, salted water until al dente then refresh and drain thoroughly.
4. Mix the pasta, cauliflower, broccoli and sauce and put in a dish.
5. Sprinkle with the remaining cheese and bake in the oven for 15minutes of until the top is browned and the pasta is hot.