Wild onion (Allium) species the world over are edible. Allium triquetrum, a native of Europe, and also common in the wilds of Britain, is the wild onion that has taken off in NZ.
(The first mention of it being naturalised here that I can find is from the 1930s. In this 1936 article it's referred to as being 'a very frequent garden escape' so - contrary to what I previously thought - it seems it may have been brought deliberately by settlers.)
You can eat all parts of the plant.
Onionweed as a substitute for other oniony things
Use onionweed just as you'd use spring onions from the store. Two simple, yummy things to do with them are:
1. Chop them up finely and mix into butter to make a garlic butter substitute. It's delicious spread on French bread and baked like garlic bread, or over fish.
2. Make a spread (especially nice on rice crackers) by mashing together chopped onionweed, soft tofu, ginger juice (from the gratings squeezed), soy sauce, and a splash of peanut oil.
I love anything that combines onionweed with ginger! This Hugh Fearnley-Whittingstall recipe has gone down a treat with onionweed used instead of spring onions. And if you don't eat pork or any meat, it's really nice with whole mushrooms and firm tofu instead.
Note that the green bits of onionweed get stringier as they get older, so chopping them fine is important if you're actually eating them, rather than just using them to flavour something and removing them after cooking.
Using onionweed flowers
Throw the pretty onionweed flowers into salads, or batter them and fry them.
If you're frying them, cut the stalk about half a centimetre down from the flowers. That's just enough to hold the flower cluster together, but isn't enough of the stalk to be stringy. Everyone in my family seems to like this onionweed tempura.
You can also steam a handful of bulbs as a vegetable, and someone recently told me he liked them chopped up in salads.
Onionweed medicinally?
Two of onionweed's cultivated rellies - onion and garlic - are known for their anti-microbial properties, as well as being useful when blood pressure is high ...
I'd love to know if onionweed has similar benefits, and if so, to what degree? And are the onion greens useful medicinally, or are most of the medicinal properties concentrated in the roots? All info gratefully received!
Onion weed links:
Onion flower tempura recipe
Blog post about flower fritters
Plants for a Future database
Google Images