Cook the couscous according to the instructions on the packet. Finely chop the onion and coriander leaves. Cut the avocado, tomatoes, bell pepper and feta into small cubes. Mix everything (the couscous can still be warm) and season with salt, lemon juice and a little olive oil.
If you don`t like coriander at all, you can of course also use parsley, but in my opinion that changes the typical taste of this dish a lot.