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Onion Masterclass

The onion, mastered.

Most savoury cooking starts with an onion. How you cut it and how far you cook it changes a dish more than almost any spice.

Onion types

Onion: root, layers and grainCross-section with numbered parts: 1 Papery skin, 2 Layers (fleshy leaves), 3 Root plate, 4 Grain, 5 Stem / neck.

Root, layers and grain

An onion is layers of swollen leaves joined at a root plate; its 'grain' runs from root to tip.

  1. Papery skin: Dry outer leaves; discard (or use for stock colour).
  2. Layers (fleshy leaves): Each ring is one leaf; they separate into petals.
  3. Root plate: Holds the layers together — keep it on while dicing.
  4. Grain: Lines running pole to pole; slicing with them keeps strands intact, across them breaks more cells.
  5. Stem / neck: Top end; cut off first.

How to cut an onion

Dice

The chef's method uses the root to hold the layers together while you cut.

  1. Cut off the stem (top) end; leave the root end on.

  2. Halve from top to root and peel off the skin and any papery first layer.

  3. Lay one half flat. Make vertical cuts from near the root to the top, without cutting through the root — close together for fine dice, wider for coarse.

  4. Optional: make one or two horizontal cuts towards the root (unnecessary for most onions; the layers already separate).

  5. Cut across the vertical cuts to release the dice, stopping at the root. Chop the root piece or save it for stock.

Best for: Curry and sauce bases, Salsas, Fried rice

Knife cut: Small dice

Slice with the grain (pole to pole)

Slices that follow the onion's natural lines from root to tip hold their shape when cooked.

  1. Halve from top to root and peel.

  2. Cut a V-shaped notch to remove the root core.

  3. Lay flat and slice from top to root along the lines on the onion's surface, angling the knife towards the centre so slices are even.

Best for: Fried onions (birista), Fajitas and stir-fries, Onions you want to stay as strands

Slice across the grain (half-moons)

Slicing across the lines cuts through more cells; slices soften and break down faster.

  1. Halve from top to root and peel.

  2. Lay flat and slice across, perpendicular to the lines, into half-moons.

  3. Separate the layers with your fingers.

Best for: Caramelising, Onions meant to melt into a sauce, Quick pickles

Rings

Whole rounds separated into rings, for frying, grilling and raw salads.

  1. Peel the whole onion, keeping it round.

  2. Cut a thin slice off one side so it does not roll.

  3. Slice across into rounds of the thickness you want.

  4. Push the rings apart; use the small centres elsewhere.

Best for: Indian onion salads (laccha pyaz), Grilled and fried onion rings, Burgers

Knife cut: Rondelle

Petals

Separated layers cut into squares or wedges, for skewers and tikka.

  1. Cut the onion into quarters or eighths from top to root.

  2. Remove the root.

  3. Separate the layers, two or three thick, into petals.

  4. Thread onto skewers between meat, paneer or vegetables.

Best for: Tikka and kebab skewers, Stir-fries (Chinese-style squares), Roasting

The science

Why onions make you cry

Cutting an onion breaks its cells and starts a two-enzyme chain reaction that releases a volatile irritant into the air near your eyes.

  1. In an intact onion, sulphur-containing flavour precursors and the enzyme alliinase are kept in separate parts of the cell.

  2. Cutting mixes them. Alliinase turns the precursor into a reactive sulphenic acid.

  3. A second enzyme, lachrymatory-factor synthase — identified by Imai and colleagues in 2002 — converts that acid into propanethial S-oxide, the 'lachrymatory factor'.

  4. This volatile gas reaches the eye's surface, irritates the nerve endings there, and the eye produces tears to wash it away.

Fewer tears

  • Use a sharp knife: clean cuts break fewer cells than crushing with a blunt blade.
  • Chill the onion for 15–30 minutes first; a colder onion releases the gas more slowly (widely recommended kitchen advice).
  • Cut near an extractor fan or open window, and keep your face back from the board.
  • Leave the root end until last — it is the most pungent part.

Sources

Cooking stages

Typical times for one or two medium onions in a wide pan with 1–2 tablespoons of fat. More onions, a crowded pan or a weak burner take longer. Browning comes from the Maillard reaction and caramelisation.

  1. Sweated · 5–8 min

    soft, no colour

    Heat
    Low–medium, lid on or a pinch of salt
    Looks
    Glossy and translucent at the edges; no browning.
    Tastes
    Sweet and mild, no sharpness.
    Use for
    Risotto, Velouté and white sauces, Delicate soups

  2. Soft / translucent · 8–12 min

    light pink-gold (Indian 'halka gulabi')

    Heat
    Medium
    Looks
    Fully translucent, just starting to turn pale gold at the edges.
    Tastes
    Sweet, rounded.
    Use for
    Everyday curry bases, Pasta sauces, Soups

  3. Golden · 12–20 min

    golden brown · bhuna base

    Heat
    Medium, stirring often
    Looks
    Evenly golden to light brown; volume reduced by about half.
    Tastes
    Savoury-sweet with toasty notes from browning reactions.
    Use for
    North Indian gravies, Korma and rogan josh, Stews

  4. Caramelised · 40–60 min

    jammy onions

    Heat
    Low–medium, stirring every few minutes; a splash of water to lift the fond
    Looks
    Deep mahogany, soft and jammy; reduced to a fraction of the starting volume.
    Tastes
    Very sweet, rich and savoury.
    Use for
    French onion soup, Onion gravy, Tarts and toppings

  5. Birista / crisp-fried · 10–15 min

    fried onions · barista · beresta

    Heat
    Shallow or deep oil at about 160–170 °C, thin even slices
    Looks
    Uniform deep golden-brown and crisp once drained (they keep darkening for a minute after leaving the oil).
    Tastes
    Sweet, nutty and slightly bitter if taken too far.
    Use for
    Biryani layers and garnish, Mujaddara and koshari, Kormas (often ground)

Hot oil: keep children away, use a deep pan no more than a third full, and never leave frying unattended. See deep-frying.

Onions across cuisines