Why It Works
- Slowly caramelizing the onions in butter until deep golden brown builds concentrated sweetness.
- Using French Le Puy lentils ensures the soup stays brothy with tender lentils that hold their shape instead of turning mushy.
My two go-to comfort soups in the dead of winter couldn’t be more different: a hearty lentil soup and a gooey, cheese-laden French onion soup. Lentils are earthy, filling, and nourishing without weighing you down, while French onion soup, when done right, is indulgent, balancing sweetness with savory broth and molten Gruyère. Rather than choosing between them, I decided to combine them. The result is this lentil French onion soup, developed by Renu Dhar in our Birmingham, Alabama, test kitchen. It’s the best of both worlds: a brothy, deeply caramelized onion soup fortified with tender French lentils—rich and cozy, but with enough backbone to count as dinner.
This soup succeeds or fails based on three things: the onions, the stock, and the lentils. Renu found that when those elements are right, the cheesy crouton top becomes a glorious finishing touch, not a distraction from a thin or bitter base.
How to Properly Caramelize the Onions
Let’s start with the onions. Two pounds may look like a heaping pile at first, but they cook down dramatically. The goal is creamy, deep golden-brown onions with a fully softened texture and a sweetness that runs through. That takes at least 45 minutes, often closer to an hour, and it can’t be rushed. The reason is that browning and caramelizing aren’t the same thing. You can crank the heat and get a dark color on the outside of the onions in 15 minutes. But that’s just surface browning. The interiors will still be firm, watery, and sharp-tasting. True caramelization—the kind that gives you jammy texture and deep sweetness—requires time for moisture to cook off and for sugars to slowly concentrate and transform. As the onions soften, their cell walls break down, their natural sugars become more pronounced, and the whole mass collapses into something silky and cohesive.
To achieve this, keep the heat at medium-low once the onions have softened, and stir frequently, scraping up the fond as it forms. If the browned bits on the bottom threaten to scorch before the onions are ready, add a tablespoon of water and scrape; it’s insurance against burning and bitterness. Starting the onions in butter is also key here. Butter’s milk solids encourage browning and add a round, toasty richness that oil alone can’t replicate.
Once the onions are caramelized, you’ll add sherry—a move that Daniel uses in his French onion soup recipe. Its nutty, oxidized flavor that pairs well with the caramelized onions. But you could also use vermouth or white wine, and some folks even use red wine or port. Take your pick—they’re all good.
Choosing the Right Stock
Next is the stock, the second most important component of this soup after the onions. Traditionally, French onion soup is made with beef stock, but making it at home is time-consuming. Store-bought versions, meanwhile, are so terrible that they’re not worth considering—unless you happen to buy your beef stock from a butcher or other store that sells the good stuff.
While homemade beef stock is the very best option, there’s no reason to turn your nose up at chicken stock, which is without a doubt the most versatile of all stocks to keep on hand in the kitchen. A great homemade chicken stock will produce an absolutely phenomenal French onion soup, and a good store-bought brand can still make a great version.
Why The Lentil Type Matters
After the stock come the lentils, and for this soup, French lentils du Puy are non-negotiable. Unlike red or brown lentils, they retain their shape and remain firm, adding texture rather than dissolving into the broth. Simmer them just until tender but intact; you want distinct lentils suspended in oniony broth, not sludge.
Building the Perfect Cheese-Capped Bowl
Finally, the gratinée. Cut your French bread slices to fit snugly inside the bowls—about 1/2 inch thick—so they float without excessive overhang. Toast them until fully crisp; this prevents disintegration once they hit the soup. A light swipe of butter and a rub of raw garlic perfume them. Nestle the slices over each bowl, overlapping slightly, then blanket them with a full 1/2 cup of grated Gruyère before broiling until the cheese is melted and blistered in spots.
Each spoonful should drag through stretchy cheese, crisp-edged bread, sweet onions, and earthy lentils that still have bite. It’s familiar but also a little unexpected—exactly what I want when it’s still freezing outside.
This recipe was developed by Renu Dhar; The headnote was written by Leah Colins.
