Combine the onions, butter and salt in your slow cooker. Cover and cook on high for 4 hours, until the onions turn a caramel brown color. Add the dry sherry and garlic, stirring to combine. Keep the slow cooker on high, uncovered, to burn off any alcohol, 10 to 15 minutes.Stir in the broths, thyme, bay leaf and sugar. Cook, covered, on high for an additional 1 to 2 hours to let the flavors blend. Remove the thyme and bay leaf and stir in soy sauce and pepper.Serve with a sprinkle of parmesan cheese and garnish with extra thyme. Enjoy!