Topset onions or walking onions or egyptian onions or perennial onions are an old style of onion, that instead (or in addition to)making flowers, produce small bulbs or bulbils at the top of 'flowering' stalks.
The ones I've grown always continue to stay green throughout summer, and only produce tiny green bulbs at the base, like a spring onion (although confusion surrounds this term, too).
I got hold of a topset that also flowers (as part of a crossing project), used my original variety, and a seedling that developed from seed i collected from my original variety when one year it produced a tiny amount of viable seed. I also got a seedling from Kelly Winterton's Green Mountain potato onion that procuced topsets.
I grew out all 4 varieties in a side by side trial.
My original and the seedling from it didn't show any difference. stayed green, produced topsets, and slightly bulbous bases.
The Green Mountain topsets are the real standout.
The topsets all developed into large pale straw coloured bulbs that ae huge for a topset onion, the size of supermarket onions. The tops senesced, but haven't really shriveled, leaving my a bit concerned about their storage ability. However not one produced any topset bulbils. These will all be replanted this autumn to see if i can get a pile of topsets next year.
This strikes me as a very useful characteristic - the bulbils are easy to look after, and robust in early growth - much as I hope the Amuri Red onions will do.