In a large sauté pan over medium heat, add olive oil to coat the pan. When the pan is hot, add the diced chicken to the pan and season with salt and pepper. Cook for 5-6 minutes or until slightly browned.
Add the onion, garlic, chili sesame oil, and hoisin sauce into the pan. Mix and cook for 3 minutes.
Add the tomatoes into the pan and the water. Cook until the tomatoes are slightly softened for another 3 minutes.
Add the cut string beans into the pan. Season with more chili sesame oil to your preference. Reduce the heat to medium low, cover and cook for 15 minutes or until the string beans are tender. Serve and enjoy!