Heat 2 tablespoons of the butter in a large dutch oven or stock pot, over medium heat. Once the butter has melted, add the onions and garlic to the pan. Cook until the onions are translucent and the garlic is fragrant.
To the pot, add the mushrooms, and cook until they have released their liquid and are tender.
Add the remaining 1 tablespoon of butter, Worcestershire sauce, parsley, bell peppers, salt and pepper. Cook for an additional 5 minutes.
Pour a little bit of the beef stock in the pan to deglaze. Scrape up and mix in any bits that are stuck to the bottom of the pot.
Add the rest of the beef stock and the roast beef to the pot, bring to a boil, and then reduce to simmer.
Add the cheese and the cream to the pot and simmer, stirring frequently, for an additional 30 minutes.