In a 5-6 quart slow cooker, combine carrots, potatoes, onion, and garlic. Put chicken breasts on top of veggies. Mix chicken stock, cornstarch, thyme, salt, and pepper with a fork and poor mixture over chicken and veggies. Cover slow cooker with lid and cook on low for 8 hours or on high for 6 hours.
With tongs or slotted spoon, transfer chicken and veggies to a deep platter. Cover platter to keep warm. Stir peas and cream into the cooking liquid and heat through. Spoon sauce over chicken and veggies on the platter.