Look at Serious Eats. There is a lengthy discussion of the variety of onions they recommend. We just made it and used all Visalias. I recommend using a slow cooker. We left them for 14 hours.
A French cultivar called "Jaune Paille des Vertus" -- a yellow onion -- is considered one of the best in that country. Unfortunately, this onion is pretty much impossible to find in the States, at least commercially.
Here in the USA, our allium selection is downright mediocre, even in major metropolitan areas and the top farmers markets.
For example, the Japanese have a dozen different bunching onions but recipes translated for Americans simply call for "spring onion" or "scallion" because the Japanese varietals can't be found in groceries and we basically only get one type of "green onion."
I suggest you use a yellow onion. More than likely, you will not have any choice of cultivar. American commercial onion farmers are probably using 2-3 cultivars, bred to favor quick growth, disease/pest resistance, size, easy harvesting, and long storage, not for flavor.
I prefer to use a mix of the yellow and the sweet variety - Vidalia if they are in season, or Texas Sweet if not. What counts most is taking the time to brown the onions slowly to a deep golden brown. I've found if you go too dark, your soup can take on a bitter edge note. The mix I use is about 1/3 yellow onions and 2/3 sweet onions - but I also like using all sweet if the onions are fresh. Mostly it's a matter of taste, some folks like it with a bitter bite, I don't. An easy way to decide if your onions are bitter or not is to soften a few of them in a pan, and give them a taste - before you slice up all the rest. If that's bitter, better to know before there is a serious pile of onion sliced.
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Here in the USA, our allium selection is downright mediocre, even in major metropolitan areas and the top farmers markets.
For example, the Japanese have a dozen different bunching onions but recipes translated for Americans simply call for "spring onion" or "scallion" because the Japanese varietals can't be found in groceries and we basically only get one type of "green onion."
I suggest you use a yellow onion. More than likely, you will not have any choice of cultivar. American commercial onion farmers are probably using 2-3 cultivars, bred to favor quick growth, disease/pest resistance, size, easy harvesting, and long storage, not for flavor.
Anyhow, best of luck.