Preheat the oven to 350 degrees F. Season the chicken with salt and pepper. Dredge in flour, shaking off excess. Reserve the leftover flour. Heat the oil in a large skillet over medium-high heat. Brown the chicken, in batches if necessary, and then divide among 2, 9 x 13-inch casseroles. Add the leeks, garlic, carrots and celery to the skillet and cook 4-5 minutes. Stir in a cup of the reserved flour. Slowly stir in the stock until well combined. Mix in the thyme and bring to a simmer. Divide and pour mixture over the chicken. (If freezing one casserole, cool it to room temperature, and then tightly wrap and freeze. Thaw in the fridge overnight before finishing as described below). Push the potatoes in and around the chicken, and then cover and cook 50-60 minutes, or until the chicken is cooked through. Uncover, sprinkle with the peas, and cook 5 minutes more.