Add the sliced onions to the bottom of the slow cooker.
Place the chicken thighs on top of the onions and season with salt, pepper, and thyme.
In a small bowl, stir together the beef broth, Worcestershire sauce, and minced garlic. Pour the mixture over the chicken and onions.
Cover and cook on low for 6 hours until the chicken is tender and the onions are soft.
Just before serving, sprinkle the shredded Gruyere cheese over the chicken. Cover for 5 minutes to let the cheese melt.
Spoon the chicken, onions, and broth into bowls and serve with slices of crusty baguette for dipping.