Make the burgers by mixing together the minced chicken, onion, garlic, rosemary and lemon zest. Season well with salt and pepper, and then divide the mixture into 4 large, thick burgers. Heat the olive oil in a large frying pan, and slowly cook the burgers on both sides till cooked through. Meanwhile, make the pesto by putting the olives, garlic, basil leaves, olive oil, mustard and feta cheese in a food processor .Pulse for only a few seconds, leaving the pesto nice and chunky. When the burgers are cooked, slice the foccacia buns in half and slightly toast them. Place a spoonful of Greek yogurt on the bottom piece of each foccacia, and arrange some salad leaves and cucumber slices on top. Next, place a chicken burger on top of each bun. Cover the burgers with a heaped tablespoonful of the black olive pesto. Place a few onion slices on top, and finally place the top piece of foccacia on. Serve the lemon and rosemary chicken burgers with chunky fries and a salad.