The Goosefoot family of plants are relatively hard to identify, particularly as there are a huge number of subspecies and some of them can hybridise freely further confusing things and that makes identification a microscope affair rather than just simply a visual affair.
However this one below is Fat- Hen ( I've crossed referenced this a lot and sought secondary identification on this one too!)
It's got that mealy appearance that fat-hen gets as it gets older, the leaves are that sort of grey/blue green and heavily toothed
It's got those rather easily identifiable flowers just before it goes to seed and this is one way I use this to ID it as the spikes of flowers are different that those on orache, a similar and easily misidentified plant with this one.
The leaves are like a gooses foot (hence the name of this genus) and are a different shape than those of Good King Henry which is another plant it can be confused with..
Tollund man was found to have eaten Fat-hen shortly before he died.
It provides a very nutritious food, very cabbagey in taste but pleasant, it does contain high amounts of oxalic acid but this is broken down with prolonged cooking, and it gives a pleasant meal similar to other beets or chards and it can also be eaten raw in salads.
It's also commonly called Lambs Quarters and a host of other local names.
Although supposedly common all over I don't find it very often here in Northern Ireland.