Preheat the oven to 200C/400F/Gas 6.
Place a frying pan over a medium heat. Once hot, place the duck breast skin-side down into the pan. Cook for five minutes, until the skin is browned and crisp.
Drain some of the fat released from the duck and then turn the breast over and cook for a further three minutes, until the flesh has browned and then transfer into the oven to roast for ten minutes, or until cooked through.
Using a 10cm/4in pastry cutter, cut out a pastry round. Brush the pastry round with the beaten egg and place onto a baking sheet. Place into the oven and bake for ten minutes, or until risen and golden.
For the sauce, place the oil into a pan over a medium to high heat. Add the mushrooms and shallots and sauté for three minutes.
Add the red wine, stock cube and butter and simmer until the sauce has thickened.
To serve, place the duck on a bed of wilted spinach, drizzle over the sauce and top with the puff pastry round.