Season the chuck steak generously with salt and freshly ground black pepper. While heating your charcoal grill to high heat (200°C/392°F), let the seasoning penetrate the meat for about 10 minutes. Once the grill is hot, grill the steak for exactly 4 minutes on each side to achieve a nice char. Remove from heat and let it rest for 5 minutes before cutting into 2-inch cubes. Save any juices that come out.
Heat 2 tablespoons of oil in a large pot over medium heat (170°C/338°F). Add minced garlic and cook for 30 seconds until fragrant, then add chopped onions and cook for 3-4 minutes until they become clear. Add tomato paste and stir constantly for 1 minute to cook off the raw taste.
Add the cubed beef and all the collected juices to the pot. Pour in the beef stock, chopped tomatoes, soy sauce, and vinegar. Add the bay leaves, brown sugar, and star anise. Let it come to a boil, then lower the heat (140°C/284°F) and cover the pot. Let it simmer gently for 1 hour, stirring occasionally to prevent sticking.
After an hour, add the whole mushrooms and continue cooking for 45 minutes with the lid on. Check the liquid level every now and then – the meat should always be mostly covered. Add the cubed potatoes and butter, then cook for another 15 minutes or until you can easily pierce the potatoes with a fork.
Turn the heat up to high and let the sauce reduce until it thickens and coats the back of your spoon. Taste and add salt and pepper as needed. Squeeze fresh lemon juice over the top and give everything a final stir. Turn off the heat and let the dish rest for 10 minutes before serving – this helps the meat soak up more flavor. Serve hot with freshly cooked rice.
Note: If you notice the sauce getting too thick at any point, add hot beef stock ¼ cup at a time. Your final sauce should be rich and thick enough to coat the meat but still be easily spooned over rice.