Sweat onion and garlic in a pot big enough to hold the cooked spaghetti. When soft, add the cream and chilli oil. Bring just to boil. Add the poutargue. When the spaghetti is cooked drain it, keeping back a little of its liquid. Add the spaghetti to the cream and poutargue, stir tenderly. Use a drop or two of the spaghetti water to dilute the sauce if needed.