Start by melting butter and olive oil in a pot over medium heat.
Add the chopped onions and a pinch of salt, cooking them until they are deeply caramelized.
Stir in the balsamic vinegar and let it cook for a minute to enhance the flavor.
Pour in the beef stock and add the sprig of thyme, bringing the mixture to a simmer.
Add the ditalini pasta and cook until al dente, following package instructions.
Once the pasta is cooked, stir in the cream to create a rich and creamy texture.
Season with black pepper to taste and remove the thyme sprig.
Serve the soup hot, garnished with chives and topped with shredded gruyère cheese.
