Melt a knob of butter in a pan and gently fry a sliced onion for a few minutes. Leave to cool. Meanwhile blanch a head of broccoli, broken into florets, by cooking in a pan of boiling salted water for 2 min, drain and run under cold water to cool, then drain well.
Unroll a sheet of Jus-Rol puff pastry and place on a baking tray. Spread with 3tbsp wholegrain mustard leaving a 2cm border all around. Scatter over the onions, broccoli, and 100g grated cheddar. Bake at 220C/Gas 7 for 15-18 min until pastry is risen and cheese melted.