
This chicken and mushroom Wellington wraps juicy chicken breasts in a creamy mushroom filling and flaky golden puff pastry. It looks impressive enough for guests, yet the method is straightforward enough for a weekend dinner or special family meal.
Make the filling: Melt the butter in a frying pan over medium heat. Add the chopped mushrooms and cook for 8 minutes, stirring occasionally, until softened and lightly browned. Stir in the crushed garlic, thyme leaves, and lemon juice, then cook for 1 minute more over medium-high heat until any excess moisture has evaporated. Season with salt and pepper, remove from the heat, and let the mixture cool completely.
Transfer the cooled mushroom mixture to a bowl. Add the low-fat soft cheese and mustard, then mix until smooth and well combined. Cover and chill while you prepare the chicken.
Place the chicken breasts on a board and gently pound them with a meat mallet or rolling pin until they are an even thickness. Season both sides with salt and pepper.
Unroll the puff pastry and cut out one 16 cm rectangle and one 18 cm rectangle. Place the smaller rectangle on a sheet of baking parchment and brush it lightly with beaten egg. Set one chicken breast on top, smooth-side down. Spread the mushroom filling evenly over the chicken, then place the second chicken breast on top, smooth-side up, with the thicker ends facing opposite directions for an even shape.
Brush the exposed pastry border and the top of the chicken with more beaten egg. Lay the larger pastry rectangle over the chicken and gently tuck it around the filling so it fits snugly. Trim the excess pastry into an oval shape, leaving a border of about 4 cm, then press the edges with a fork to seal. Use any pastry scraps for decoration if desired. Chill the Wellington for at least 1 hour, or up to 24 hours.
Preheat the oven to 210C (190C fan/gas 6 1/2) with a baking tray inside. Brush the chilled Wellington all over with most of the remaining beaten egg. Carefully transfer it on the parchment to the hot baking tray and bake for 25 minutes.
Remove the Wellington from the oven, brush with the remaining beaten egg, and sprinkle lightly with sea salt if desired. Return to the oven and bake for 10 to 15 minutes more, until the pastry is crisp and golden brown and the chicken is cooked through. Rest for 5 minutes before slicing and serving.
Cool the mushroom filling fully before assembling so the pastry stays crisp. If your mushrooms release a lot of liquid, cook them a little longer until the pan is dry. The Wellington can be assembled a day ahead and baked just before serving.