Which are the best onions to use in onion soup? Chowhounds actually have thought about this question a lot, and the consensus is: plain old yellow cooking onions.
The key to great onion soup is not using expensive, specialty onions. The requirements are a long, low, slow caramelization of the onions and a good stock. chazzerking says specialty onions like Walla Wallas and Vidalias lose their sweetness differential after caramelizing. cmkdvs tried sweet onions and found them to be much too sweet after caramelizing.
Karl S caramelizes in a rectangular nonstick electric skillet for best temperature control and no scorching.
Large onions are preferable to small ones, just because they’re easier to slice.
Board Link: Best onions for onion soup?