Brush some mustard over each steak, then place in the middle of a pastry square. Top each steak with a heaped tablespoon of the onion jam or chutney, brush egg wash on the pastry edges and cover with another pastry square. Press the edges to seal, then trim the excess pastry to leave a 1cm border and crimp with a fork. Cut a little cross in the top of each parcel and brush with egg wash.
Cut out 4-5 pastry hearts per Wellington. Add to the top of each, avoiding covering the vent, and brush with the egg wash. Place the Wellingtons on the tray and bake for 12 minutes for medium-rare meat. Remove from the oven and leave to rest for 5 minutes. Serve with a salad or blanched veg.