Rinse the chicken both inside and out under cool running water. Pat dry. Season the chicken liberally both inside and out with salt and pepper. Place in a slow cooker. Scatter the onions, carrots, celery, garlic, thyme, and lemon juice over top of the chicken. Cover the slow cooker and set the temperature to high. Cook for 6 hours, undisturbed.
Remove the chicken from the slow cooker, and pour the accumulated juices into a 2-cup heatproof container. Skim the fat from the top and transfer the liquid to a small saucepan. Dissolve the cornstarch in a small bowl with 2 teaspoons of water and whisk to form a slurry. Whisk the slurry into the juices and bring to a boil over high heat. Cook until thickened, season with salt and pepper, and continue to cook an additional 4 to 5 minutes. Cut the chicken into pieces and serve, with the thickened pan juices ladled over the top.