Step 1: Cook the onion
Heat the oil in a pan and cook the onion gently for 8–10 minutes until soft. Transfer to a bowl and leave to cool.

Step 2: Flatten the chicken
On a cling film-covered board, place one chicken breast and cover with another sheet of cling film. Gently bash with a rolling pin until about ½cm thick. Repeat with the rest.

Step 3: Prepare the filling
In a bowl, mix the cooled onion, sausagemeat, chopped herbs, and breadcrumbs.

Step 4: Assemble the roulade
On a large double layer of cling film (about 30x40cm), lay out the prosciutto slices to cover the surface. Arrange the flattened chicken breasts on top, fitting them together and seasoning with salt and pepper.

Spread the sausage mixture evenly over the chicken, then loosen the cranberry sauce with a spoon and spread it on top.

Starting from one long edge, use the cling film to help you roll up the chicken and fillings tightly into a spiral, making the prosciutto the outermost layer. Wrap the roulade tightly in cling film, twist the ends to seal, and freeze for 30 minutes.

Step 5: Roll out the pastry
Dust your work surface and roll out one block of puff pastry to about the thickness of a £1 coin. Place on a baking tray lined with parchment paper.

Unwrap the chicken roulade and place it on the pastry. Trim the pastry to leave a 2–3cm border around the roulade. Keep the off-cuts for decorating.

Step 6: Encase the roulade
Roll out the second pastry block to the same thickness. Brush the edges of the base pastry with the beaten egg wash. Lay the second pastry over the roulade, gently pressing out any air pockets. Trim the edges, leaving about a 2cm border.

Step 7: Seal and decorate
Crimp the edges by pinching with your fingers or pressing with a fork (dust the fork with flour if pastry sticks). Brush the whole wellington with egg wash. Use reserved pastry scraps to decorate the top or score a pattern with a sharp knife.

Chill the wellington covered for at least 1 hour or up to 48 hours. It can also be frozen at this point for up to 2 months — defrost in the fridge before baking.

Step 8: Bake
Preheat the oven to 200°C (180°C fan) / gas 6. Brush the wellington with any remaining egg wash and bake for 1 hour 15 minutes, until golden and crisp. If frozen, check the center is piping hot by piercing with a knife.

Serve warm with roast potatoes, vegetables, and gravy if you like. Have some kitchen paper handy, as chicken releases juices when sliced.