Heat the oil and butter in a large heavy saucepan and gently fry the onion until soft. Stir in the garlic and marjoram leaves, the pumpkin and potato, and continue to cook for a minute. Season with the chilli, salt and pepper. Add enough stock to just cover the pumpkin, turn down the heat, and simmer until the pumpkin is tender, about 20-25 minutes. You may need to add more stock.