Preheat the oven to 400 degrees F. In a large saucepan, over medium-heat, melt the butter. Add the onions. Season with salt and pepper. Saute the onions until slightly caramelized, about 8 to 10 minutes. Add the garlic, sherry, Worcestershire sauce and stock. Season with salt and pepper. Bring the liquid to a boil and reduce to a simmer. Cook the soup for 15 minutes. Ladle the soup into oven-proof deep bowls. Spread the each piece of bread with 1/4 cup of the rillette. Float the croutons on top of the soup. Cover each crouton with a piece of cheese. Sprinkle the cheese with parsley. Place the bowls on a sheet pan and place in the oven. Bake until the cheese is bubbly and golden, about 6 to 8 minutes. Remove from the oven and serve.