How to Caramelize Onions (Low, Slow, Deeply Brown)

To caramelize onions, slice them thinly and cook them slowly in butter or oil over medium-low heat for 45 to 60 minutes, stirring regularly and adding splashes of water to lift browned bits, until they are soft, jammy and deep brown. Expect about 1 kg (2 lb) of raw onions to cook down to roughly 1 to 1.5 cups.

What is actually happening

Despite the name, caramelized onions are not only about caramelization. Onions contain roughly 4 to 8% sugars (mostly glucose, fructose and sucrose) plus amino acids and sulfur compounds. Long, slow heat drives off water, concentrates those sugars, and triggers two kinds of browning:

• Caramelization: sugars break down and recombine into hundreds of brown, toasty, slightly bitter compounds. Fructose starts to caramelize at a lower temperature (around 110 °C / 230 °F) than sucrose (around 160 °C / 320 °F).
• The Maillard reaction: sugars react with amino acids to make savoury, meaty, roasted flavours.

Water is the brake. While the onions are full of moisture, their temperature stays close to 100 °C / 212 °F, and browning is slow. As water evaporates over the first 20 to 30 minutes, the temperature on the onion surfaces and pan bottom climbs, and browning accelerates. That is why the colour seems to do nothing for ages, then change quickly.

Steps

1. Halve the onions through the root and slice them pole to pole. Slices 3 to 5 mm thick along the grain (root to tip) hold their shape better than rings and cook more evenly.
2. Heat butter or oil in a wide, heavy pan over medium heat. Use about 1 tablespoon (15 g) fat per 500 g onions. A mix of butter and oil gives butter flavour with less risk of burning. A wide pan matters: more surface means faster evaporation.
3. Add the onions with a pinch of salt and stir to coat. The pan will look overloaded. That is fine; onions shrink by about three quarters. Salt draws out moisture through osmosis, speeding the softening stage.
4. Cook until softened and wilted, about 10 minutes. Stir occasionally. The onions turn translucent and release liquid. This is essentially sweating.
5. Lower the heat to medium-low and cook, stirring every few minutes. Spread the onions in an even layer between stirs so they have time to brown on the bottom.
6. Deglaze with a splash of water whenever brown fond builds up. When a brown film forms on the pan, add 2 to 3 tablespoons (30 to 45 mL) of water and scrape. The fond is concentrated flavour; dissolving it coats the onions and keeps the pan from scorching.
7. Keep going until the onions are deep brown and jammy, 45 to 60 minutes total. Look for the colour of strong tea or milk chocolate and a soft, spreadable texture.
8. Season to taste and use or store. Add more salt, a splash of balsamic, sherry vinegar or wine, or fresh thyme. A little acid brightens the sweetness.

Stages and uses

Faster options (with trade-offs)

• Baking soda: about 1/8 teaspoon per kg onions raises the pH. Maillard browning runs faster in less acidic conditions, so colour comes sooner. The alkalinity also breaks down pectin, giving softer, almost melted onions and a slightly soapy taste if you overdo it.
• Higher heat with frequent water: cook over medium-high and add water every few minutes. It works in 20 to 30 minutes, but needs constant attention.
• Oven or slow cooker: big batches can cook in a covered Dutch oven at 200 °C / 400 °F for an hour, then uncovered on the stovetop to reduce and brown.

Chef Fizz says: Patience is the secret ingredient. Turn the heat up to rush them and you get burnt edges with raw middles.

Common mistakes and fixes

• Burnt in spots, pale in others: the heat is too high or the pan is too thin. Lower the heat, use a heavier pan, and deglaze more often.
• Onions stay pale after 30 minutes: they are still too wet or the heat is too low. Raise the heat slightly and stir less often so the bottom can brown.
• Bitter flavour: some onions scorched. Remove any black bits; deglaze sooner next time.
• Stringy onions: you sliced across the grain into rings. Slice pole to pole.
• Mushy, soapy onions: too much baking soda. Use only a pinch, or skip it.

Storage

Cool, then refrigerate in a sealed container for 5 to 7 days. Freeze flat in a bag or in an ice-cube tray for up to 3 months; a single cube is perfect for a sandwich or a pan sauce.