Lightly coat a larger slowcooker with avocado spray. Heat the olive oil in a large pan over medium. Add the carrots, leek, and onion, and cook until the vegetables are beginning to soften ~ 8-10 minutes. Stir in the garlic, salt, and pepper and cook 30 additional seconds. Remove and transfer to the bottom of the slow cooker.
Add the rinsed brown rice to the slow cooker. Arrange the chicken thighs on top, then pour in 8 cups of the bone broth. Add the bay leaf, thyme and rosemary.
Cover and cook on low for 6 hours or high for 3 1/2 hours, until the rice is tender and the chicken is cooked through. To prevent the chicken from becoming dry, check it early. (The chicken is done when it registers 165 degrees F at the thickest part.) If the chicken finishes cooking before the rice, remove it first so that it does not overcook, then re-cover the slow cooker and continue cooking until the rice is tender. Remove the bay leaf and herb stems and discard.
Place the chicken on a cutting board. Shred the chicken, then return it to the slow-cooker. Stir in the lemon juice. If you'd like the soup thinner, add additional water until you reach your desired consistency. Taste and adjust the seasoning as desired. Serve warm, sprinkled with additional fresh thyme.