Trim any fat from the pork, cut the meat into 1 cm (½ in) thick medallions and place them in a mixing bowl.
To make the marinade, remove the leaves from the sprig of rosemary, chop them roughly and add them to the pork with the garlic, lemon zest, olive oil and pepper. Cover and chill for at least 1 hour – overnight is best as it allows the flavours to permeate the meat.
Heat the oven to 180°C (350°F, gas mark 4). Thirty minutes before you want to serve, core the apples and cut a ring through the skin around their circumference, so they can expand as they cook. Tuck a sprig of rosemary into the centre of each apple, with a teaspoon of caster sugar if using, and bake the apples for 25 minutes, or until just soft.
Meanwhile, heat a ridged, iron grill pan or nonstick frying pan over a medium-high heat. Remove the medallions from their marinade, season with salt and sear them for 2-4 minutes on each side until browned. Serve with the baked apples and garnish with sprigs of rosemary.
Variation: make a simple gravy by reducing some chicken or pork stock. You could also deglaze the grill pan or frying pan with a little dry cider.