Chef Thomas Keller’s Sous Vide Onions Recipe

Chef Keller finds that 85ºC is the perfect temperature for cooking all vegetables sous vide. Specific vegetables will vary in cooking times based on their density, porousness, and size, but the 85ºC mark is optimal for breaking down the fibers in plants. In fact, if the water bath is less than 83ºC, you could cook vegetables “until doomsday,” as Chef Keller jokes, and they would remain firm.

For fibrous or root vegetables over ½ inch in thickness, begin testing for doneness after 35 minutes. For more delicate vegetables, due to variations in vegetable composition and density, it will take some experimentation on your part.

It’s important to understand the density of specific vegetables before cooking them sous vide: carrots and fennel bulb are both dense vegetables, but the fennel bulb has space between its leaves; asparagus is porous and less dense. Onions are softer and less dense; for example, it’s also possible to make caramelized onions sous vide by adding unsalted butter and a bit of balsamic vinegar into the sous vide bag. All of these factors, along with the size of the vegetable or the size to which they are cut, play into how long it takes each sous vide vegetable to cook.

When it comes to food safety, vegetables being cooked sous vide should be peeled or scrubbed well. Any portions of vegetables that may harbor soil (e.g., the roots of a head of garlic or an onion) should be removed; they pose a greater risk of botulism and listeria, both of which are very dangerous in an anaerobic environment such as sous vide.