Boil the pasta, onion and broccoli stalks in plenty of lightly salted boiling water for about 7 mins, then add the florets and cook for another 3 mins. Reserve about 400ml of the water, then drain the pasta and vegetables.
Return the reserved water to the pan and dissolve the stock cube, whisk in the mustard and crème fraîche, then season to taste. Bring to the boil. Stir in the drained pasta, vegetables and half the cheese, mixing until melted. Stir in the parsley.
Heat the grill for 3 mins. Tip the pasta and vegetables into a shallow, ovenproof dish. Mix together the remaining cheese, breadcrumbs and herbs, then scatter on top.
Stand the dish on the base of a grill pan and grill for about 3 mins, turning if it starts to brown. Keep watching so the top doesn’t start to burn or the cheese will toughen. Remove and leave to stand for 5 mins before serving.