Mangeout:
I was using lettuce as short for ‘miner’s lettuce,’ I know real lettuce does not belong in this discussion.
As P. ‘umbraticola’? Mrs. Foolonthehill says no.
Im a landscape guy so I just take a sample to a local plant nursery & ask them.
I was going to say Polygonaceae, Mangetout, and it looks like you may have nailed it. I have one reservation: is the sample a lot mure succulent than it appears on the picture, cause the pic of Claytonia looks pretty succulent. Other than that, sure!