Add the butter to the pan and melt over medium-high heat. Add the shallot and garlic and cook until fragrant, about 30 seconds.
Add the mushrooms and season with salt and pepper to taste. Cook until softened, about 3 minutes.
Add the cognac and cook until completely evaporated. Stir in the mustard and cream. Cook for 1 minute.
Add the beef stock, Worcestershire sauce, chopped scallions, parsley, and hot sauce to taste. Season with salt and pepper.