French onion soup

How long to caramelize onions for French onion soup

About an hour, and there is no real shortcut. Recipes that promise caramelized onions in 15 minutes are describing browned onions, which taste of char rather than of the sweet, jammy depth this soup is built on. Properly caramelized onions are soft all the way through and a deep, even amber. Three pounds of raw onions will cook down to about two cups.

Start with medium heat so the onions release their water and wilt, then turn the heat down and let them cook slowly. Stir every five minutes or so. Each time, scrape up the brown film forming on the bottom of the pot. That film is where most of the flavor is. If it starts to look dark rather than brown, add two tablespoons of water and scrape. The water lifts it off the pot and back onto the onions.

Does baking soda speed it up?

Yes. A pinch (1/8 teaspoon) added with the salt raises the pH, which speeds up browning and cuts the time by about a third. The trade-off is texture. The onions break down into something closer to a paste than soft ribbons. For this soup, where the onions should still have some shape in the spoon, it is better to give them the full hour.

French onion soup in the crock pot

A slow cooker cannot brown onions the way a pot does, but given enough time it caramelizes them, with almost no stirring. The onions come out softer and sweeter, closer to jam, with less of the roasted edge. It is a good trade for a day when you cannot stand at the stove for an hour.

• Caramelize: put the 3 lb of sliced onions, the butter (melted or in pieces) and the salt in a slow cooker of at least 5 quarts. Cover and cook on low for 10 hours, stirring once or twice if you are around. They will be soft, golden and sitting in a lot of liquid.
• Reduce: prop the lid open with a wooden spoon, or take it off, and cook on high for 1 to 2 hours more, stirring now and then, until most of the liquid has gone and the onions are a deep amber brown.
• Finish: stir in the sherry for the last 15 minutes of reducing, then the flour, then the broth, thyme and bay leaf. Cook on high for 1 hour, until hot and slightly thickened. Remove the herbs and season with the vinegar, salt and pepper. Top and broil the bowls as in the recipe.

Overnight works well: start the onions before bed and they are ready to reduce in the morning. The full soup fits a 6-quart slow cooker. The finished soup is the same per bowl, about 385 kcal with the bread and cheese.

Which onions to use

Ordinary yellow onions. They have enough sugar to caramelize well and enough sharpness to balance it. Sweet onions such as Vidalia tend to make a sweeter, flatter soup, and red onions turn a muddy color. Slice them pole to pole, root to tip, rather than across. Slices cut that way hold together better during the long cooking.

The broth matters

Use low-sodium beef broth. The soup simmers for a while, and the cheese and bread both add salt, so a regular broth often ends up too salty to eat. Good chicken broth also works and gives a lighter soup, and the strained liquid from a pot of braised beef or braised short ribs, with the fat lifted off, is better than either. For a vegetarian version, use a mushroom broth and add a teaspoon of soy sauce for savory depth; the porcini soaking liquid from our mushroom soup is a good start.

The splash of sherry vinegar at the end is not traditional, but it is important. Caramelized onions are sweet, and a little acid keeps the soup from tasting like onion jam.

The best cheese for French onion soup

Gruyère is the classic: nutty, a little sweet, and it melts into a smooth, browned crust. Comté and Emmental are the closest substitutes. Swiss and provolone are cheaper and melt just as well but taste milder; a mix of Swiss with a little grated Parmesan gets closer to Gruyère. Avoid pre-shredded cheese if you can. The anti-caking starch on it keeps it from melting into a smooth layer.

Without broiler-safe bowls

A day-old sourdough loaf makes the best toasts, because it stays firm in the broth. Put the toasted bread slices on a sheet pan, pile the cheese on them, and broil until bubbling. Float one or two on each bowl of soup. You lose the cheese melting down the sides of the bowl, but the soup is otherwise the same.

Keeping it

The soup without the bread and cheese keeps 4 days in the fridge and freezes for 3 months, and it tastes even better on the second day. Caramelize a double batch of onions, in two pots or in a wide Dutch oven over a longer time, and freeze half. Next time, the soup takes 30 minutes.

What it is worth nutritionally

About 385 kcal a bowl, with 16 g of protein, about half of it from the Gruyère. Most of the calories come from the cheese, butter and bread. The soup underneath is about 190 kcal a bowl. If you want it lighter, use one toast and half the cheese per bowl, which saves about 100 kcal. Sodium is the number to watch with this soup. Low-sodium broth makes the biggest difference, much more than cutting back the salt you add to the onions.