Heat olive oil over medium heat in a deep skillet (that has a tight fitting lid). Add in white onion and saute until soft and fragrant. Add in the finely diced garlic and saute another 1-2 minutes (do not brown).
Add in the stock and parsley. Season with salt and black pepper to taste. Bring to a boil and add the clams then the wine. Cover with tightly fitting lid and boil away until the clams open shaking the pan every now and then. Once all the clams have opened, pour the whole pan into a serving dish. Squeeze the lemon over the clams. Sprinkle with zest, green onion and parmesan cheese and serve with hot crusty bread.
Tip: When buying clams or other shellfish make sure that they are not open at all as these are not good to eat. Also, if after the majority of the clams have opened you have one or two that do not open, discard them as they are not good either.