The flag was inventer in 1925 and have a feeling that stripes were worn before that date but doubt that the flag is based onthe shirt.
As for onions, my Grandfather remembers them from the 1950’s when he worked on the railways in southern England, as they cycled from town to town selling onions and garlic. He said that they sold several varieties of each.
In a typical French supermarket you have a choice of maybe 6 types of onions and several types of garlic while I’m sure that it’s less in the average market in the UK (at least it is in Ireland).
I prefer French garlic to the stuff I buy in Ireland. It’s much sweeter and never have smalled cloves within the main clove.
By all means check out this link from my earlier post, which answers some of your questions.
Bear in mind that this trade dates from the early years of the 20th century, when the vast majority of fruit and vegetables available in Britain were native and seasonal, and there was, naturally, considerably less variety – in contrast to today when almost any commodity from anywhere in the world is available year-round.
It was well worth it for the Bretons to make the short trip on the ferry across the channel to a market more than ready to pay a little more for good-quality produce not otherwise available. Bear in mind as well that there was a certain caché in buying from the French onion-seller, a touch of the cosmopolitan in the largely hum-drum life of the average housewife. Good marketing in other words.
There was a programme on BBC TV a few years ago about French onion sellers. It said that many growers are now going over to the production of asparagus , instead of onions. Apparently they can get more for this crop , it suits the sandy soil where they used to grow the onions and they don’t have to go to all the expense of coming over to Britain to sell it .
We still get them around here occasionally, but I think they now tend to wear the striped shirts because they are playing up to the stereotype, rather than some unbroken tradition.