Preheat the oven to 375 degrees F.
For the stock: In a large stock pot, combine the chicken, onions, carrots, celery, garlic, salt, pepper, bay leaves, thyme and water. Season heavily with salt. Place the pot over medium heat and bring to a boil. Reduce heat to medium low and simmer for about 2 hours, or until the chicken is tender. Remove the pan from the heat and cool the chicken completely in the cooking liquid. Remove the chicken and strain the liquid, set aside. Remove the skin, fat and bones from the chicken. Medium dice the chicken meat and set aside.
For the dressing: In a saute pan, over medium heat, melt the butter. Add the onions and celery. Season with salt and pepper. Saute for 3 to 4 minutes, or until wilted. Add the garlic and continue to saute for 1 minute. Remove from heat and pour into a large mixing bowl. Add the bread cubes, sage, parsley, and egg. Mix well. Season with salt and pepper. Add the diced chicken meat. Stir in enough of the stock, so the mixture is very wet, about 3 to 4 cups. Allow the dressing to sit for 30 minutes. **Add more of the stock if needed. The mixture should still be very wet after it has set. Butter a deep casserole baking dish and pour in the mixture. Bake for about 1 hour, or until bubbly and golden brown. Spoon onto serving plates and garnish with parsley.