How to chop carrots and sweat onions

Kitchen masterclass: as part of our new collaboration with Leiths School of Food and Wine we are going back to basics. In the first of the series, we show you how to chop core root vegetables properly and learn how to sweat onions, which is the a ingredient to a large number of savoury dishes


Onions are included in many savoury dishes. Yellow onions are the most versatile; white and red onions are valued for their mildness. Shallots and spring onions, also called salad onions, are members of the onion family too. Shallots are valued for their mild, sweet flavour and are used whole in casseroles as well as sliced and chopped for all manner of dishes. Choose firm onions with a thin, papery skin. Photography: Peter Cassidy.


Preparing onions or shallots for slicing and dicing

1. Cut a small slice off the top of the onion so it can stand upright. Trim a little off the hairy part of the root but keep the root intact as this holds the onion together when you are cutting it 


2. Stand the onion with the trimmed top surface down. Using a large, sharp knife (a small knife for shallots), cut down through the onion to halve


3. Peel each onion half and discard the skin. It is also a good idea to remove the first of the inner pale leaves of the onion as these tend to be leathery and do not break down during cooking


Slicing onions

1. Place the onion rounded-side up on the board with the root end furthest from your chopping hand. With your hand in a claw shape and the tips of your fingers bent, hold the onion lightly.Your thumb will support the root end


2. Slice the onion in a rocking action, keeping the tip of the knife in contact with the board as you draw it down to cut through the onion


3. As you reach the root end of the onion, turn the onion on to the largest flat side and slice again, ensuring no onion is wasted.


Dicing onions

1. Halve and peel your onion. With the flat side down and the root end away from you, slice through the onion vertically, towards the root but not right through it (to keep it intact). For fine dice ensure the cuts are close together


2. Slice horizontally through the onion once or twice, again not right through the root, but very close to it, keeping the knife slightly angled towards the board for safety


3. Now move the onion so that the root end is on your left and proceed as for slicing an onion. It may be a little more difficult, but try to hold the onion together in your other hand ‘claw’ to protect your fingertips and fingernails
Kitchen skills with to dice an onion

Cooking onions

When onions are used to flavour a dish, they are usually sweated – cooked gently in a little butter or oil. We lay a greaseproof paper “cartouche” on top of sweating onions to minimise evaporation. Similarly, we place one over braising vegetables and poaching fruit to keep them immersed in the liquid or fat. Dampening the cartouche helps to maintain a moist environment. Dry cartouches are used too, when baking pastry blind, for example.


Making a cartouche

1.To make a cartouche, cut a square or round of greaseproof paper and fold into segments, the shape of an elongated triangle. Trim the triangle to a fraction larger than the radius of your pan


2. Unfold the paper into a circle that should fit snugly inside the saucepan on top of the onions


3. Crumple the cartouche and dampen under cold water before use. The crumpling helps the greaseproof paper hold a little more water. Lay on top of the ingredient(s) to be cooked


Sweating onions

This technique describes the cooking process of softening an onion and drawing out its natural sweetness without allowing it to take on any colour. The onions are gently sweated in a little oil or butter. Using a dampened cartouche helps the sweating process and seals in the juices.

1. In a suitably sized saucepan, melt a nut of butter or a little oil. Put the onions in the saucepan and place a dampened cartouche on the surface, in contact with the onions. Cover with a tight-fitting lid, place over a very gentle heat and allow the onions to ‘sweat’.

2. Check the onions occasionally, especially if a lot of steam is escaping. If the cartouche is dry, re-dampen it and return it to cover the onions. If any onions have browned on the bottom of the pan, don’t stir them in. Discard them and use a clean saucepan to continue sweating.

3. After 10-15 minutes, check the onions again. They will be ready when they have lost volume and become translucent. If you squeeze a piece of onion between your fingers there should be no resistance. If you taste a piece, it will have a sweet, mild flavour




This versatile root adds sweetness to stocks and stews; is steamed, sautéed or roasted as a side dish; or served raw grated in salads. Carrots work well with parsley and thyme, and aromatic spices such as cumin, coriander and cinnamon. Choose firm, unblemished carrots.

Blocking Carrots

Wash and peel the carrot and trim off the top end. Cut the carrot into 4 finger-width pieces. Cut off a side of the carrot. Turn it and repeat on all other sides to create a rectangular block of carrot (Reserve any trimmings for stock)


Cut the blocked carrot into batons or sticks, 5–6.5cm long and approximately 1cm square. For smaller batons, or allumettes, cut each large baton lengthways into 4 thinner sticks, about 5mm square


Cut the blocked carrot into very thin slices (1–2mm thick) and stack them neatly. Slice through the carrot as uniformly as possible to create julienne

Dice and brunoise

Cut across batons to form uniform dice (left);the carrot should be perfectly square in shape. Cut across allumettes to form brunoise (right). Cut across julienne to form fine brunoise (top)

Leiths How to Cook by Leiths School of Food and Wine (Quadrille), £30
To learn these and other essential skills, visit and for more hands on cookery classes with the Leiths chefs, go to