Slice the carrots and celery root. Cut the carrots into thin circles and set them aside. Peel the celery root, slice it into circles, then cut each circle into bite-size pieces.
Prepare the potatoes and leek. Peel the potatoes and cut them into quarters. Slice the leek into circles and rinse well to remove any grit.
Cut the chicken. Cut the chicken breasts into small rectangular pieces. Season them with salt, pepper, and a drizzle of oil, then toss to coat.
Layer the bottom. Spread the leek circles across the bottom of the baking dish. Top with the celery root pieces. Toss the potatoes with oil and salt, then arrange them over the celery root.
Add the chicken and carrots. Place the seasoned chicken on top of the potatoes. Tuck in the bay leaf, then cover everything with the carrot circles.
Add water and bake. Pour in the warm salted water, then cover the dish first with a sheet of baking paper (parchment), pressing it lightly down toward the edges, and place aluminum foil on top, crimping it tightly around the dish. Bake at 400°F (200°C) for 60 to 70 minutes, until a fork slides easily into the potatoes and the chicken is cooked through
Finish with the cream. Pour the cream into a bowl. For a thicker stew, stir in the flour or cornstarch in small doses until smooth. Remove the foil, pour the cream gently over the stew, and stir lightly. Return the dish to the oven, uncovered, for 5 to 10 minutes, just until the stew begins to bubble.
Serve. Take the dish out of the oven and let it rest for a few minutes. Remove the bay leaf, taste and adjust the salt and pepper, then ladle the stew into warm bowls and serve warm.