Heat 1 tablespoon of oil in skillet and brown chicken pieces. Transfer chicken to a dish and keep warm. Add the other teaspoon of oil to the pan along with the sliced onions. Cook over low heat for 10 minutes. Add garlic and cook for another 5 minutes or until onions start to brown. Add the chopped tomatoes and liquid, Kalamata olives, oregano, rosemary, salt and pepper.
Place chicken pieces in the sauce. Cover and bring to a boil. Reduce to a simmer and cook for 40 minutes. Arrange feta on top of the chicken. Cover and cook for another 15 minutes until chicken is done and cheese is melted.