Light a grill or preheat the oven to 500°. Using a small, sharp knife, peel the lemon, removing all the bitter white pith. Cut the lemon into 1/4-inch dice, discarding the seeds. Transfer the diced lemon to a medium bowl and add the tomatoes, cucumber, shallot, thyme and chives. Stir in the 1/4 cup of olive oil and season with salt and pepper.