Soak your clay pot in water for 15-30 minutes if it's unglazed. Drain thoroughly. In a large bowl, toss the chicken pieces with salt and pepper. If desired, heat 1 tbsp olive oil in a separate skillet and sear chicken briefly until lightly browned on all sides, then transfer to the clay pot. Add remaining olive oil to the pot (or skillet, then transfer) and sauté chopped onion, minced garlic, chopped carrots, and chopped celery until softened, about 5-7 minutes. Return the chicken to the clay pot. Stir in the crushed tomatoes, chicken broth, dried thyme, and bay leaf. Add the diced potatoes. Cover the clay pot with its lid and place it in a cold oven. Set the oven temperature to 350°F (175°C) and bake for 1.5 to 2 hours, or until the chicken is tender and cooked through and vegetables are soft. Carefully remove the bay leaf before serving. Garnish with fresh chopped parsley and serve hot.