For the tomato-cucumber salad:
1) Smash the garlic clove, sprinkle with 1 tsp of salt and with the flat side of a large knife, mash and smear to a coarse paste. Put the garlic paste in a large bowl and stir in the olive oil and lemon juice. Add the tomatoes, cucumber, pepperoncini, and dill and toss. Give a couple of generous grinds of black pepper. Toss again, and set aside.
For the chicken:
1) Preheat a griddle pan or nonstick frying pan to medium high.
2) Spray the chicken breasts lightly with the olive oil and season with salt and black pepper. Griddle the chicken, in batches if necessary, to avoid crowding the pan, turning once, until cooked through, about 2 mins per side. Place each breast on a plate and serve topped with the tomato-cucumber salad.