In a 4- to 5-quart slow cooker combine Brussels sprouts and onion. Sprinkle with salt and pepper. Pour cider over sprout mixture.
Cover and cook on low-heat setting for 3 to 3 1/2 hours or on high-heat setting for 1 1/2 to 2 hours.
In a small bowl combine melted butter, maple syrup, and thyme. Pour over sprouts in cooker, stirring to combine. If desired, garnish with additional fresh thyme leaves.