I use regular brown or yellow onions and cook them very slowly--first they steam in their own liquid
covered, until very tender. Then when they're nice and reduced they start to brown a little on the bottom. Scrape the browned bits into the onions and let the bottom brown again, repeating until they are an even brown. You may have to add a little water if they really start scorching.
Done slowly like this, I don't think it matters which onions you use.