Preheat the broiler. In a stockpot or Dutch oven, melt 3 tablespoons butter over medium heat. Add the onion and garlic and cook until softened, about 10 minutes. Add the beef broth and thyme, bring to a boil, then reduce the heat and simmer for 10 minutes. Toast the baguette under the broiler until lightly browned; remove and spread with butter.
Remove the soup from the heat, add the vinegar and season to taste with salt and pepper. Serve with the Parmesan and the toast.