Light a grill or preheat a grill pan. Using a sharp knife, peel the grapefruit and lime, removing all of the bitter pith. Cut in between the membranes and release the sections onto a work surface. Squeeze the membranes over a medium bowl to extract the juices. Stir in half each of the olive oil, cilantro and mint and season with salt and pepper. Add the chicken and toss to coat.
Grill the chicken thighs over moderately high heat, turning them occasionally, until they are lightly charred outside and cooked through, 8 to 10 minutes.
Meanwhile, coarsely chop the grapefruit and lime sections and transfer to a bowl. Add the red onion and the remaining olive oil, cilantro and mint. Season with salt and pepper. Serve the chicken thighs with the citrus salsa.
The salsa can be refrigerated for up to 3 hours.
A citrusy Albariño, a mojito (of course) or a hefeweizen (a wheat beer often served with lemon).