Spray a 6- to 7-quart slow cooker with cooking spray. Add onions, butter, salt, pepper and dried thyme. Whisk flour, sherry and water together until blended and stir into onion mixture.
Cover and cook on Low 9-10 hours, until onions are softened and deep brown.
Stir broth into slow cooker. Increase heat to High, cover and cook 10 minutes until hot.
Meanwhile, broil baguette slices about 3 inches from heat for 2 minutes, turning once, until lightly browned. Top evenly with Gruyère and broil about 1 minute longer, until bubbly.
Top each bowl with 2 cheese toasts and fresh thyme if desired.