One of the most important ingredients to all savoury dishes from just about every culture, the onion is the quintessential base and starting point for the cook.
There are a huge number of varieties, big and small and with various strengths but I have listed here the four I use most commonly.
These are the most common of all the onions and can be used for just about anything. They come in various strengths, the larger varieties often being the mildest.
Again, a very common onion, these are a little sweeter than the yellow onions. As such, it makes them ideal for Indian curries in particular, though they are also nice in salads.
The Roscoff onion is my absolute favourite. It is probably the most versatile onion and is as happy in Asian cooking as it is in the classic French Onion Soup. If you keep no other onion, keep this.
Also in the same family, there are Shallots, which I deal with separately!