After researching everyone's ideas, I confirmed that this is French Sorrel, also known as Rumex scutatus, buckerleaf sorrel, or garden sorrel.When cultivated, sorrel takes 60 days from seed to salad and is a perennial. It must be used fresh.
It's a cleansing herb when eaten fresh in the spring, with a tart, fresh, lemony flavour.
Be aware that, like spinach, sorrel contains oxalic acid and should be eaten in moderation if raw. It might also be best to avoid it entirely if you are suffering from arthritis or kidney disease.