These days, whenever I go hiking in Golden Gate Park or the Presidio of San Francisco, I'm always coming up on fields of these circular green leaves with tiny flowers in the middle. Called miner's lettuce, they are an edible salad green that can be eaten raw in salads, like nasturtiums, or boiled/sauteed like spinach.
Also known as winter purslane and Indian lettuce, miner's lettuce was a staple food for the gold miners during the Gold Rush because it is plentiful and contains large amounts of Vitamin C and helped to prevent scurvy. Miner's lettuce is a spring plant native to the mountains and coastal areas of Western North America, from Alaska to California. The circular leaf is mounted on a thin stem. Often there will be tiny white flowers in the center of the leaf. All parts of the plant are edible. It tastes somewhat like spinach when cooked.
As with any foraged food, you should be sure you aren't picking from an area that has been sprayed with chemicals or has sewage underneath the ground.
(Image: Kathryn Hill)