Light a grill. In a mini food processor, pulse the pitted olives with the crushed garlic and oregano until chopped. Add the 1/4 cup plus 2 tablespoons of olive oil and pulse until finely chopped. Season with pepper.
Brush the tomatoes, onion and cut sides of the rolls with olive oil. Grill the tomatoes and onion over high heat until they are softened and lightly charred, about 2 minutes for the tomatoes and 6 minutes for the onion. Transfer to a plate and season with salt and pepper. Grill the bread until lightly toasted, about 2 minutes.
Season the chicken cutlets with salt and pepper and grill them over high heat, turning occasionally, until they are lightly browned in spots and cooked through, 5 to 6 minutes. Cut the chicken cutlets to fit the toasted rolls and top with the sliced tomatoes, sliced onion and olive relish. Close the sandwiches, cut them in half and serve right away.