Valerianella locusta, aka mâche, lamb's lettuce

"I'd give my left ball for some mâche right now"
— one of my chef customers

This is one of those "weeds" that crops up in the [allotment garden], in cornfield margins and open meadows from time to time. It's a pretty little rosette that at first glance could be spinach, and indeed when picked young has many of the same qualities. It has a delicate, tender juicy leaf up to about 6" (15cm) long and is ideally suited to salads, having a mild, faintly nutty flavour.
It's native to Europe and parts of Asia and remains extremely popular in France. Growing it is easy if the climate is cooler; in the UK I could have it almost year-round provided I protected it from a freeze. It's not tolerant of high temperatures, and certainly I don't know anyone growing it in California's scorching Central Valley summers. The fragility of the plant explains its scarcity given that machine picking is difficult, and harvesting is usually done by hand. This makes it expensive and that makes it "gourmet" and in high demand by chefs and gastro-poseurs.
One of my pet peeves (I have many vegetable-related) is customers asking for "mâche lettuce". It's not lettuce. Being in the [honeysuckle] family, at best it's several distant relationships away; like escarole, frisée and spinach, they are not lettuce. Neither can you cross radishes with beets, people. You can mix it with lettuce (and I can recommend it) but they are [umpteen|umpteenth cousins] at best.
The names are intriguing. I knew it as corn salad, and indeed it was often to be found in wheat fields, or at the least in the margins. "Lamb's lettuce" because the leaf allegedly resembles a lamb's tongue, and "mâche" is basically French for "chewed up" (think [papier mâché]), possibly suggesting its fragility. There are doubtless twenty-nine other country names for it as well in every country. It seems that (in the US at least), mâche is the most popular name, probably because the French are the largest producers of the stuff, and it sounds exotic.
When I grew it, I'd sow it in rows in the spring, and again in early autumn. It was very keen to go to flower (especially if it got hot) so I'd probably aim for a couple of plantings after the first. The flowers are quite pretty, creamy white trumpets with five petals. Pretty as they are though, the sight of them was enough to make me stop eating that crop and move to the next. It did seem to affect the delicate flavour enough to have me stop. In time, had I left it to seed it might have naturalised in the garden in the way that my rocket and chard did from year one.
Personally I'd no more cook it than I would lettuce, but apparently there are those who lightly braise it. For me, it was always a salad green, either by itself or combined with baby lettuce, a little rocket or spinach. A light dressing was sufficient for me, so as not to overpower the delicacy of the flavour. I also have to admit that goutweed ([ground elder]), it was one of those things I'd munch on whilst gardening. There's nothing like putting the [hoedow|hoe down] for a moment to have a light, nutty snack.

I want to do this again. I miss growing my own food.
[https://www.specialtyproduce.com/produce/Mache_518.php]

corn salad (thing)