1. Preheat your oven to 400°F or 200°C.

2. Slice the roasted beef into thin strips.

3. In a large skillet, melt 30 grams or 2 tablespoons of butter over medium heat.

4. Add the sliced mushrooms and cook until they release their moisture and start to brown, this will take around 5 minutes.

5. Add the sliced beef to the skillet and cook for a further 2-3 minutes, until the beef is nicely coated in the mushroom juices.

6. Season the mixture with salt and pepper according to your taste.

7. In a separate pan, melt the remaining 15 grams or 1 tablespoon of butter over low heat.

8. Add the chopped parsley and cook for 1 minute, until it becomes fragrant.

9. Serve the beef and mushroom mixture hot, garnished with the parsley butter.