Season the beef with salt and black pepper.
Heat the oil in a medium pan over medium-high heat. Quickly brown the beef strips, working in batches if needed. Remove and set aside.
In the same pan, sauté the onion until soft and translucent. Add the garlic and cook briefly.
Stir in the tomato paste, ketchup, and mustard until well combined.
Return the beef to the pan and add the mushrooms, if using.
Lower the heat and pour in the cream. Stir gently and simmer for 5–7 minutes, until the sauce thickens slightly.
Taste and adjust seasoning if needed. Serve hot.
