Pat the chicken dry with paper towels. Season the cavity with salt and pepper.
In a large bowl combine the onion, lemon, garlic, half the thyme, and 1 1/2 tablespoons olive oil; season with salt and pepper and mix it well. Stuff the bird with the mixture. Using 2 (3-foot) pieces of kitchen twine, tie up the chicken: Tuck the wing tips between the wings and the body. Put the midpoint 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 chicken, breast side up in a roasting pan, and layer the strips of bacon up breast until covered. Scatter over the remaining thyme, drizzle over some more olive oil and season everything with salt and pepper.
Roast the chicken for 1 1/2 hours; arrange the mushrooms around the chicken 45 minutes after roasting. Baste with the pan juices every 20 minutes. The chicken is done when an instant-read thermometer registers 165 degrees F when inserted into the thickest part of the thigh (the legs should wiggle easily in their sockets.)
Remove the chicken from the pan, cover with foil, and let stand for 10 minutes to rest. Serve the chicken with the roasted mushrooms and pan juices.