Cut the pumpkin into wedges or chunks (with or without skin which ever you prefer).
Place the lemon juice and zest, rosemary and oil into a large freezer bag then add the pumpkin. Coat the wedges with the oil mixture, fasten the bag and leave to marinate for a couple of hours.
Preheat the oven to 190℃ /Fan 170℃ . Tip the marinated pumpkin in a single layer onto a baking tray or ovenproof dish. Sprinkle with a little sea salt and black pepper. Roast in the preheated oven for about 45 minutes, depending on the size of the wedges, or until the flesh is soft and starting to caramelise around the edges. Turn the pumpkin mid way through cooking.