Stock: Fill a Dutch oven or large pot ¾ full with water. Place the pot over medium heat. Drop the thighs into the water with the onion, 1 handful of the carrot and 2 handfuls of the celery. Add the peppercorns and thyme sprigs and bring to a low simmer for 50-minutes. Note: Once bubbling, lower the heat to medium-low.
Shred: Use a pair of tongs to pluck the chicken out of the pot. Allow to cool on a cutting board then shred or cut into small pieces. Use a strainer or skimmer to remove the vegetables, thyme, and peppercorns. Discard these and return the chicken to the stock.
Cook Orzo: Raise the heat to high and when the stock is boiling, add the orzo and cook, stirring occasionally for 5 minutes. Add the remaining carrot and celery and season with salt and pepper. When the vegetables are tender, turn off the heat and layer in the lemon juice, to taste, with the spinach and parsley. Ladle into bowls and serve with crusty bread.