In a large pot, In small, heavy-bottomed pot, arrange chicken breasts in a single layer. Add water, salt, peppercorns, and bay leaf.
Bring to a boil, skimming scum that floats on top. Lower heat, cover, and cook in a barely a simmer for about 10 to 15 minutes or until thermometer inserted in the thickest part of chicken reads 165 F.
With a slotted spoon, remove chicken from heat and allow to cool to touch. Coarsely shred chicken and keep warm.
Using a fine-mesh sieve, strain liquid. Discard aromatics and reserve about 4 cups of the broth.
In a large pot over medium heat, heat oil. When Add onions, garlic, celery, and carrots. Cook, stirring occasionally until softened.
Add reserved broth (from poaching chicken) and bring to a boil.
With hands, break rice into individual grains. Add to pot and stir to distribute.
Lower heat, cover, simmer, stirring occasionally, for about 30 to 40 minutes or until the rice has softened and the congee has thickened to desired consistency.
Add shredded chicken and cook for another 8 to 10 minutes or until heated.
Season with salt and pepper to taste. Ladle to serving bowls and garnish with green onions, if desired.