1. Slice the beef into thin strips, about 6 mm or 1/4 inch thick.

2. Peel and chop the onion into small pieces.

3. Slice the mushrooms into thin strips.

4. In a large pan, melt 30g/2 tbsp of butter over medium-high heat.

5. Add the beef and cook until browned, seasoning with salt and pepper to taste.

6. Remove the cooked beef from the pan and set aside, leaving the juices behind.

7. Add 15ml/1 tbsp of water to the pan, scraping the bottom to release the browned bits, and set the liquid aside.

8. In the same pan, melt an additional 30g/2 tbsp of butter over medium heat.

9. Add the chopped onion and sliced mushrooms, cooking until they are tender.

10. Combine the cooked beef, onion, and mushrooms in the pan.

11. Add the reserved juices, 30ml/2 tbsp of soy sauce, 5ml/1 tsp of mustard, and 30g/2 tbsp of tomato puree.

12. Bring the mixture to a boil, then reduce the heat to low and simmer for 10 minutes.

13. In a small bowl, mix 15g/1 tbsp of flour with a little cold water until smooth.

14. Stir the flour mixture into the sauce and continue to simmer for another 5 minutes.

15. Sprinkle chopped parsley and halved cherry tomatoes over the top of the dish.

16. Serve hot with steamed rice, pickled cucumbers, and green beans.