I've been using an online Paella recipe that instructs you to sweat some onions for 5 minutes, then add garlic for a few minutes, followed by the vegetables, tomatoes, rice and stock.
Is there a reason the onions need to be added and fried first (and before the garlic)? Does it affect the taste at all?
Recipe was Saffron Seafood Paella
It's not so much the taste as the texture. If they haven't been sauted first, the onions stay relatively crunchy during the rest of the cooking. The same is true of the garlic, but you'd usually have cut the garlic into much smaller pieces so it doesn't take as long to soften up, hence kicking the onion off first and adding the garlic a bit later.
When you prepare the onions first you bring out the sugars of the onion by carefully caramelizing it. The same with the garlic, but it needs less heat (and therefore is added after the onions) If you put it in with the other vegetables the onion will be cooked. It will still be sweet, but not caramelized.
This method is not especially for Paella, it is used in countless recipes.
Well, I'm Spaniard myself and have never used onions to cook Paella or eaten one with it.
The thing about frying onions is not only the texture change but the sugar and juices it releases, that makes the fat where you fry it have a more "sweetness" taste.
If you don't fry Garlic, it can have a very bitter acrid taste to it. Frying it help sweeten the taste.
If the recipe calls for onions then it's not paella. A real Spanish paella never ever contains onion. The reason given is that the onion will make the rice pass the desired point of done-nes. Personally, I don't believe that, as other vegetables can form part of a paella (like bell peppers).
Anyway, Vicky's answer is correct, you want the onions (or bell peppers) to be crunchy and that's why they have to be added first.
