Peel the onions and cut into fine rings (works quite well with a cucumber slicer). Quarter, core and wash the peppers. Cut into strips as thin as possible with a sharp knife.
For the marinade: mix 125 ml of olive oil with 3 tablespoons of herb vinegar. Season with salt, pepper, sugar and freshly chopped thyme. Mix the onion rings and the pepper strips well with the marinade. Let it steep for at least 1 hour in a cool place.