The Best Onion Slice Thickness For Perfect French Onion Soup

If you've had an exceptional bowl of French onion soup, you know the difference optimally sliced and caramelized onions can make: Imparting just the right amount of sweetness, nutty flavor, and creaminess in every bite. To find out the ideal thickness for perfect French onion soup, we asked Chef Randy Feltis, author of "Katherine Wants: The Ultimate Date Night Cookbook," to show us the way. "You do not want to go too thin," the co-creator of the playful husband-and-wife TikTok duo, @‌KatherineWants, told Tasting Table. "You want to go a ¼ of an inch because they'll disappear in the soup if they are too thin." 

Cooking homemade French onion soup requires sauteing sliced onions in butter until golden, then simmering them with wine and beef stock for an hour (or more) before serving with toasty bread and grated Gruyere cheese. It's a straight-forward recipe, but onions are the star of the show, so you need to pay them extra attention. For best results, slice onions with enough depth to harness the most caramelized taste and melt-in-your mouth goodness, without dissolving into the pot. You don't want to slice the onions too thick either, something Feltis notes "shows immaturity" in cooking the dish. "No one wants to take a bite of a big chunk of onion," he says.

When making simple, yet flavor-packed French onion soup, it's key to recognize and avoid the mistakes everyone makes when caramelizing onions, including slicing them paper thin or cooking with high heat, since this will make them susceptible to burning. Charred onions can turn the entire soup bitter and astringent. So, like Feltis says, slice your onions a ¼ of an inch thick, then be patient as the sugars caramelize slowly — while the sugar and amino acids in the onions brown, in a process known as the Maillard reaction. Don't skimp on the butter, either, since the flavorful fat is also a key factor in slow, even cooking. 

But overall, don't sweat the soup recipe (let your onions do that). Once you master foolproof caramelized onions, you'll be well on your way to a great bowl of soup. From there, you can get creative with your additions. For instance, you can add maple syrup to the mix for extra sweetness. In his French onion soup recipe, Feltis adds brandy and wine to deglaze the pan, plus garlic or beef bones. You can serve your soup with toasty sourdough bread like the chef, toasting the bread with cheese before serving it on top of the hot soup for extra melty goodness, or swap in croutons for extra crunch. Just remember, adequately sliced onions are key. The soup's accompaniments, like any good relationship, should enhance the recipe.