Combine the oil and flour in a medium pot and set over medium heat. Cook, stirring frequently, until the mixture is golden brown, about 3 to 4 minutes. Add the onion, celery, bell pepper, garlic, cumin, cayenne and thyme and cook 2 to 3 minutes more. Mix in the tomato paste, and then very slowly mix in the diced tomatoes. Add the stock and rice and bring to a simmer. Simmer the soup until the rice is cooked through, about 20 minutes. Add the okra and cook 3 to 4 minutes more. Thin the gumbo with a bit more stock, if too thick, and then serve.