In the same pot, melt the butter over medium heat.
Add the diced onion and cook for about 3 minutes, or until tender.
Stir in the minced garlic and cook for 1 minute, or until fragrant.
Add the all-purpose flour to the onion and garlic mixture.
Whisk until a smooth paste forms.
Gradually pour in the beef broth while whisking continuously to prevent lumps.
Stir in the half and half, Worcestershire sauce, and Dijon mustard.
Season with salt and black pepper to taste.
Bring the soup to a boil.
Reduce the heat to a simmer.