• Heat a tbsp. oil and a knob of butter in a frying pan. Add the meat and brown on all sides over a low heat.  This will take 10-15 mins. Season and leave it on a plate to cool completely.
• Meanwhile, soften the finely chopped onion in a frying pan containing 1 tablespoon of oil. After 5 mins, add the finely diced mushrooms and gradually add the chopped spinach. Cook for 10-15 mins until all the water has evaporated and the mixture is relatively dry; season and leave to cool completely.
• Unroll the puff pastry and spread half of the spinach mixture across the centre. Untie the roast, place it on top (with the rounded side downwards), and cover with the rest of the spinach. Wrap everything loosely in the puff pastry; cut off any excess. Decorate with strips of pastry across the top. Cover and leave to rest in the fridge for at least 30 mins.
• Preheat the oven to gas mark 4, 180°C, 350°F. Brush the pastry with the beaten egg. Roast in the oven for 25 to 30 minutes.
• Leave to rest for 5 mins, then slice and serve with the béarnaise sauce (optional).

Chef's Tip:

To be able to cut neat strips the puff pastry needs to be cold; put it back in the fridge for 5 minutes if necessary.