Capers

Capers are the brined flower buds of the Capparis spinosa bush. This low growing bush is native to the Mediterranean. Capers are frequently paired with lemon in sauces. The smallest of the buds are the nonpareil from France. The largest are referred to as caper berries and are frequently brined with the stem still attached.

If you don't have or don't like capers you can substitute chopped green olives (leave whole for a martini garnish)
OR Pickled nasturtium seeds
OR to vary the flavor use chopped pickles