Prepare Ingredients and Chicken: Heat a non-stick skillet over medium-high heat, add 2 tablespoons of olive oil, and sear the chopped chicken until golden brown. Place the browned chicken, carrots, celery, onion, garlic, rice, and thyme into the slow cooker. Dissolve the stock cube in 1.25 litres of boiling water and pour the stock into the slow cooker.
Cook Soup: Set the slow cooker on high for 3 hours or on low for 5 hours. The rice will absorb the broth and become tender.
Add Cream and Spinach: Once the rice is tender and the chicken is cooked through, turn off the slow cooker. Stir in the single cream and baby spinach, then replace the lid to allow the spinach to wilt.
Prepare and Add Mushrooms: In a non-stick pan, heat 2 tablespoons of olive oil over high heat. Add the quartered mushrooms and sear until golden brown. Add the butter and chopped rosemary, sautéing until the butter browns (about 1 minute). Stir the sautéed mushrooms and browned butter into the soup. Serve hot with chunks of crusty bread.