Corn salad, mache, lamb's lettuce, field salad, field lettuce (Valerianella locusta) are all names for a salad green of the valerian family.
The plant grows in a rosette of long spoon shaped leaves which may also be clustered in loose heads. Lamb's lettuce is used in salad mixes together with other greens such as mustard leaves, rocket, dandelion, frisee, etc. It can also be cooked as for spinach and used in soups and stuffings. (The lamb's lettuce name comes from its resemblance to the size and shape of a lamb's tongue!). This green has been used for thousands of years.
With a fingerlike shape, velvety feel, and mild taste, it is usually sold bunched together with its roots, at an expensive price due to its delicate and perishable nature.
CDC.gov - 5 a Day