Gather your equipment: a slow cooker, a large skillet, chopping board, sharp knife, and a large spoon. Prepare all ingredients for easy assembly.
Dice the chicken thighs into bite-sized pieces and season lightly with salt and pepper. Chop the bell pepper and onion into small, even pieces. Slice the sausage into thick rounds.
In the skillet, sauté the chopped peppers and onions over medium heat for about 5 minutes until they smell fragrant and become slightly softened, with edges just beginning to brown. Transfer them to the slow cooker.
Add the chicken pieces and sausage slices to the skillet. Sear them for a few minutes until they develop a light golden crust, then transfer everything to the slow cooker.
Sprinkle the Cajun seasoning over the meat and vegetables in the slow cooker. Pour in the chicken broth and diced tomatoes if using, then stir everything together to combine evenly.
Cover the slow cooker and cook on low for 6 to 8 hours, or on high for about 4 hours. The aroma will fill your kitchen with smoky, spicy scents as the ingredients soften and meld.
About 30 minutes before serving, add the sliced okra and a teaspoon of file powder. Stir gently, then uncover and increase the heat to high to help the gumbo thicken slightly, letting it bubble gently.
Check the gumbo’s aroma — it should smell smoky, spicy, with a hint of tomato, and the chicken should be tender and juicy. Taste and adjust seasoning if needed.
Once ready, turn off the slow cooker and let the gumbo rest for about 10 minutes; this helps the flavors meld and the broth thicken slightly. Serve hot, garnished with chopped green onions if desired.