Combine all ingredients in a 6- or 7-qt. slow cooker. Cover and cook on high until rice and vegetables are tender, 5-6 hours. Discard bay leaves and thyme sprigs before serving. If desired, remove leaves from remaining thyme sprigs and sprinkle over soup when serving.
Test Kitchen tip
There's a fair amount of chopping for this recipe, but the vegetables can be chopped in advance to reduce the time needed for last-minute prep.