Brunoise is a French cooking term meaning to cut a vegetable into small cubes of precise and uniform measurement.
A regular brunoise gives you cubes 3 mm (1/8th inch) in size; a fine brunoise gives you cubes 1.5 mm (1/6th inch) in size. In France, the fine brunoise is the standard.
Brunoise-cut vegetables can be used in sauces, such as a tomato concasse, or as an aromatic garnish on a soup or consommé.
Items to which a brunoise cut is commonly applied include carrots, celery, leeks, onion, potatoes, tomato and turnip. It is more difficult on the softer vegetables such as tomatoes.
To do a Brunoise, the first step is to peel and wash the vegetable, then regularize its shape into a rectangle or square by topping and tailing it and squaring off the sides. Then cut it into 5 cm (2 inch) long pieces, then cut each of those pieces into 4mm (1/8th inch) thick slices. Stack those slices, then cut them lengthwise into 4mm (1/8th inch) wide sticks. Then stack the sticks about three to four high, and using your knuckles as a guide for the blade of the knife, slowly and deliberately slice off pieces 4mm (1/8th inch) wide. That will give you your brunoise cubes.
For a fine brunoise, replace the 4mm (1/8th inch) dimensions in the instructions above with 1.5 mm (1/16th inch).
A classic brunoise consists of carrot, onion, leek, celery, and sometimes turnip, gently cooked in butter. If someone refers to "a brunoise" without specifying, this is what is meant. This mixture is used as an ingredient in forcemeats, salpicons, sauces, and soups.
Vegetable pieces leftover from the cut (i.e. the parts that you topped and tailed and squared away) can be used in soups or stocks, etc. The peelings can be used as stock.
When used as a garnish on a soup or consommé, the cubed vegetable is blanched very briefly in boiling, salted water, then plunged into in salted ice water to stop the cooking instantly, then drained well.
This cut was reputedly popularized in the "Brunoy Commune", which was 19 km (12 miles) southeast of Paris before Paris grew. Thus its name, brunoise, from "Brunoy."