French onion soup benefits from patient cooking and selecting the right onions. Yellow onions are traditional due to a sharp flavor that mellows and endures long simmering. Sweet onions provide greater sweetness, a milder profile, and caramelize well, producing the expected finished-soup traits. Sweet onions also reduce eye irritation when being cut. Using a single onion variety yields consistent, repeatable results. Combining varieties—such as Spanish, sweet, and yellow onions—introduces more complex flavors and textures for a more adventurous final soup.
This answer surprised us a bit as, traditionally, what makes French onion soup unique is the use of yellow onions. They're considered a multipurpose variety, with a sharp flavor profile that tends to mellow down as the onions are cooked over a stretch of time. They don't mind simmering for an hour, so it makes sense why they'd be chosen for the classic French soup.
According to Feltis, however, the sweet onion is the secret top choice because "it offers more sweetness, mellow flavor and caramelizes up nicely." In fairness, those are the exact traits you're essentially expecting from a finished French onion soup. Thanks to their milder nature, another bonus for using sweet onions in the soup is that these types of allium are less likely to make you cry when you're cutting them.
Using only one type of onion in the soup yields a pretty consistent result, and that's definitely the path to choose if you're aiming for a tried-and-tested soup that tastes the same every single time you make it. But, if you're feeling a bit adventurous, a mix of varieties could be the way to go. Chef Feltis' recommendation is to use a combination of three onions, sharing, "I like the different flavours and textures [of] Spanish, sweet and regular yellow onions
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