For boiled onions it is simply when they are soft, though how soft you want them is up to you. If you are making onion soup you want them very soft and a good 10 minutes boiling should be sufficient.
For hotdogs, onions can be boiled for about 3 – 5 minutes, and then simply kept warm. They will be soft, but will have a little bite too.
Onions in dishes using minced meats are mostly fried, but also we fry onions for other dishes and to have on their own.
Place a tablespoon of oil in a frying pan or skillet and for extra flavour, a knob of butter. Cook the onions on a low to medium heat. You have a choice how you want them to turn out:
For flavours that are characterised by other ingredients – meat for example, cook the onions until they become translucent.
For darker flavours more caramelisation is needed, so cook them until they start to darken in colour.
Avoid cooking the onion so much that the edges burn, this will add a bitter note.
Onion starters for dishes
Classic French cooking often starts with a mirepoix, which is equal quantities of onion, celery and carrot all chopped small. This is then cooked for 15 – 20 minutes on a very low light to avoid any browning at all. The idea is to release the sugars in the mix, thus sweetening it. In English we refer to the vegetables cooked in tis was as ‘sweating’, which is not sweating in the hot day sense, it really is a derivation of sweetening.
In the United Stares, particularly the Southern states they have a similar method called the Holy Trinity, which is Onion, Bell peppers and Celery.