STEP 1 Cook the pasta, adding the broccoli for the final 4-5 mins and cooking until tender. Drain well, then heat the grill.
STEP 2 Heat the butter in a saucepan and stir in the flour. Cook for 1 min, then gradually add the milk, stirring well between each addition. Bring to the boil, stirring, then simmer for 2 mins, before stirring in the mustard, half the cheese and seasoning.
STEP 3 Mix the pasta and broccoli into the sauce and spoon into an ovenproof dish. Scatter over the remaining cheese and place under a hot grill for 3-4 mins until golden and bubbling.