Finely chop onion, gherkin and capers and mix with good mayo - I use organic. Leave to stand for an hour or two in the fridge, if you have time. If the capers are preserved in salt, wash them first.
I read about making Tartare Sauce in someone's blog, but I can't remember whose, or even what blog site it was on, so I'm afraid I can't credit it, sorry. Actually, theirs may have had more ingredients, but I find the ones I use sufficient. It's even fine without the onion (as I found out tonight when I discovered I had no onions).