Preheat oven to 350˚F.
Place a rack in the middle of the oven. Coat a 9x13-inch baking dish with cooking spray.
Add the cream of chicken soup, cream of celery soup, and water to the baking dish and whisk together until combined.
Stir in the rice, vegetable medley, and half of the onion soup mix (4 tablespoons) until combined.
Sprinkle the chicken breasts evenly with salt and pepper. Nestle the chicken into the rice mixture.
Sprinkle the remaining onion soup mix evenly over the chicken. Cover with aluminum foil.
Bake until the chicken is cooked through, or an instant-read thermometer registers 160°F when inserted into the thickest part of the chicken, and the rice is tender, 50 to 60 minutes.
Remove from the oven and let rest for 5 minutes. Uncover and fluff the rice with a fork. Top with chives (if using) and serve.
