Heat 10-inch cast iron skillet over medium heat. Drizzle with 1 tablespoon olive oil, and add sausage. Cook about 7 minutes, stirring occasionally, until sausage lightly browns; spoon into slow cooker. In same skillet, add 1 tablespoon olive oil. Add onion and celery. Cook until crisp-tender, stirring occasionally; add to slow cooker. In same skillet, add 1 tablespoon olive oin and chopped chicken. Cook until no longer pink, stirring occasionally, about 7-10 minutes; add to slow cooker. In same skilet, add 1 tablespoon olive oil and shrimp. Cook until shrimp is no longer pink, about 3-5 minutes; add to slow cooker. Add okra to slow cooker. In same skillet, add 1/4 cup olive oil over medium-low heat. Using whisk, beat in flour; add Cajun seasoning and thyme. Continue to beat and cook 15 to 20 minutes, making roux that is brown and smells very nutty. Add beef stock, and beat well with whisk. Heat to boiling, beating constantly. Cook until thickened; add to slow cooker. Add diced tomatoes to slow cooker; stir well. Cover; cook on Low heat setting 6 hours. Just before service, sprinkle file powder on top of gumbo. Serve gumbo with rice. Garnish with green onions before serving.