Meanwhile blanch the broccoli by cooking in a pan of boiling salted water for 2 minutes, then drain and run under cold water to cool. Drain well on kitchen paper
Unroll puff pastry sheet and place on a lined baking sheet. Spread with the mustard leaving a good 2cm border all round. Scatter the onions, broccoli and grated cheese, still leaving the boarder
Brush edges with the beaten egg and bake for 15 -18 minutes until pastry is risen, golden and the cheese has melted. Serve hot