Prepare the chicken thighs by trimming excess skin and fat, washing, and patting dry. Season both sides of the thighs with salt and pepper and set aside.
Add all dry seasonings to a small mixing bowl: garlic powder, onion powder, paprika, and Italian seasonings. Mix well. Generously season chicken with seasoning mix on both sides.
Heat the oil over medium-high heat in a large cast-iron skillet. Once the oil is hot, add chicken skin-side down. Sear until golden brown, 3-5 minutes. Flip and continue cooking for two more minutes. Transfer to a clean plate.
Add celery, carrots, onions, potatoes, and garlic to your 6-quart slow cooker, then add the seared chicken on top of the vegetables. Add the herbs.
Add butter to the same cast-iron pan, let it melt, and then add the flour. Mix well until flour is fully incorporated into the oil, 1-2 minutes.
Next, add chicken broth and continue stirring until slightly thickened, 2-3 minutes. Season it with salt and pepper to taste, then add the sauce to the slow cooker.
Cover the slow cooker and cook (2-4 hours on the High setting or 4-6 hours on the Low setting) until the chicken and potatoes are tender.
Transfer the chicken, vegetables, and some sauce to serving plates. Garnish with herbs if desired and serve.