Wipe the pork and cut into 8 x 25 - 40 mm (1 - 1 1/2 in) thick medallions. Flatten each with the palm of your hand, and tie round with string. Season with black pepper and leave to come to room temperature. Just before cooking season with salt.

Heat 25 g (1 oz) butter and 30 ml (2 tbsp) olive oil in a heavy frying-pan over medium heat and fry the medallions for 7 minutes, turning once. Heat the grill to high.

Mix the Gruyere cheese and Dijon mustard. Spread over the pork medallions.

Brush the grid of the grill pan with oil. Place the pork medallions on the grid and glaze under the hot grill for 1 minute, or until golden. Serve immediately.