Heat the olive oil in a large enameled cast-iron casserole. Add the onion, cover and cook over low heat, stirring occasionally, until softened and just starting to brown, about 20 minutes. Uncover and cook, stirring, until browned, about 10 minutes. Add the ground walnuts, pomegranate molasses, brown sugar, saffron, cumin and the chicken broth and bring to a boil. Simmer over low heat for 15 minutes. Add the chicken thighs, cover and simmer until cooked through and tender, about 15 minutes.