Finely chop the onion and cucumber, then mix everything together. Season if necessary.
I always prepare the dressing when I go to the grill and don`t want to grind loads of ingredients for the burger.
I make my burgers as follows: The hamburger buns (briefly grilled or in the microwave at home for 15 seconds), meat, dressing, possibly cheese, lid on, done.
Tastes phenomenal and has a bit of the Big Mac in it.