4 tbsp unsalted butter

5 large yellow onions, thinly sliced

1 tsp salt

1 tsp granulated sugar

2 garlic cloves, minced

32 oz low-sodium beef broth

24 oz low-sodium chicken broth

1 tbsp Worcestershire sauce

2 sprigs fresh thyme

1 bay leaf

1/2 tbsp balsamic vinegar

Pinch of salt and pepper

5 slices French bread

2 tbsp olive oil

1 1/2 cups shredded Gruyere

1/4 cup shredded Mozzarella

2 tbsp grated Parmesan