The flavour of this recipe really varies depending on the ingredients. If your lemon has a thicker skin with thick white pith, it could be a bit bitter. A tiny pinch of sugar will balance this out...or cut the pith from the lemon, reserving peel. You can vary the amount of garlic depending on how much you like it. And make sure to buy fresh walnuts and store them in the freezer! They shouldn't be overly bitter.