Heat oven to 220C/200C Fan/gas 7. Cut a slit halfway in each chicken breast, then put each one between two pieces of baking parchment and bash with a rolling pin to flatten slightly.
Cut your sheet of pastry in half widthways and put both halves on a baking tray. Sit a chicken breast on top of each half, then cram the cavities with the Boursin (don’t worry if a bit of cheese oozes out). Season, scatter over the lemon zest then fold the edges of the pastry in to the centre and pinch shut. Flip each parcel over so the seal is on the bottom. Brush with a little oil, then bake for 30 mins until deep golden.
About 10 mins before your parcels are ready, steam or boil the greens until tender. Toss in the oil and season. Serve alongside your chicken parcels with lemon wedges for squeezing over.