These little dark green leaves are known by various rather different names: in English the most common name is lamb's lettuce; in French, it's mâche; in Hochdeutsch (standard German) it's Feldsalat, or field salad. I think the Schwiizerdütsch (Swiss German dialect) name for it is the best though: Nüsslisalat - little nut salad, since it does have a subtle nutty taste. It grows wild in our garden, where we can usually pick a big bagful a few times a year from the many weedy spots. It's very easy to grow from seed too. The flavor is enhanced by frost, like other dark greens such as komatsuna, so it's best to sow this in late fall/autumn and enjoy them in the early spring.
Unlike other salad greens such as arugula, watercress or even mizuna, lamb's lettuce does not hit you with its bite; it has a very subtle taste. I like it best on its own, though goes very well in a mixed salad too. Here it's dressed with a subtly nutty oil that enhances its flavor. This is a perfect foil by the way, for the very rich mac and cheese  in the previous post.
If your lamb's lettuce is not hydroponically grown or prewashed, wash it very carefully in several changes of water, since it grows close to the ground and can be gritty. Dry carefully with several paper towels or clean kitchen towels.
For the dressing: make a oil and vinegar dressing with an 8 to 2 ratio of light walnut or hazelnut oil to lemon juice - as opposed to the usual ratio of 7 to 3. Whisk these together (or shake together in a jar) with salt and pepper to taste. You do not want mustard in this dressing since it can overwhelm the flavor of the lamb's lettuce, and go easy on the salt and pepper too.