Bring a pot of water to a boil; add chicken and cook until no longer pink in the center, 20 to 25 minutes. Transfer chicken to a work surface and cool until easily handled.

Heat olive oil in a skillet over medium heat; cook and stir onion for about 3 minutes. Add red bell pepper; cook and stir until onion and pepper are softened, about 10 minutes. Add garlic; cook and stir until fragrant, about 1 minute. Cover skillet and cook onion mixture until slightly browned, 5 to 10 minutes.

Shred chicken using your hands and sprinkle into the onion mixture. Add sherry and capers; season with salt. Cook and stir chicken mixture until heated through and flavors have blended, 10 to 15 minutes. Mix cilantro and tomatoes into chicken mixture; cook and stir until tomatoes start to break down, 5 to 10 minutes.