How much lemon zest do you use when I do not have a lemon?

If you don't have a lemon, you can substitute lemon zest with other citrus zests like lime or orange, using the same amount as called for in the recipe. Alternatively, you can use a small amount of lemon juice (about 1 teaspoon) or vinegar for a similar tangy flavor, though it won't replicate the zest's texture. If neither is available, you might consider using a bit of citric acid as a last resort.