Heat the oven to 400 degrees F.
Pat the chickens dry with paper towels. Season the cavities with salt and pepper. In a large bowl combine the onion, lemon, garlic, 1 bunch thyme, bay leaves, and 3 tablespoons olive oil; season it with salt and pepper and mix it well. Stuff each bird with half the mixture. Using 2 (3-foot) pieces of kitchen twine, tie up each chicken: Tuck the wing tips between the wings and the body. Put the mid point of the twine under the chicken, bring the ends up and around the wings, and pull them tight against the body. Bring the ends of the twine up underneath the legs, wrap the string around them, pull the legs together, and tie them tightly. Place the chickens breast side up in a roasting pan, drizzle over some olive oil, and season them with salt and pepper. Toss the mushrooms around the chicken, scatter over the remaining thyme, drizzle over some more olive oil and season everything with salt and pepper. Roast the chickens for 1 to 1 1/2 hours; baste them with the pan juices every 20 minutes. The chicken is done when a thermometer reads 165 degrees F when inserted into the thickest part of the thigh (the legs should wiggle easily in their sockets.) Remove the chickens from the pan, cover them with foil, and let them stand for 10 minutes to rest. Serve 1 chicken with the roasted mushrooms and pan juices. Refrigerate the other chicken for another meal.