Soak the shiitake mushrooms in warm water for 20 minutes. Remove them, dice them, and reserve the soaking liquid.
Prepare all ingredients before you start cooking. Separate the whites and greens of the spring onions. Mix the cornflour with 1 tablespoon of water.
Heat a wok over high heat until it's just starting to smoke. Add peanut oil and swirl it around. Add the beef mince, flatten it out, and leave it for 2-3 minutes without moving it to get a crust on one side. Add the garlic, white parts of the spring onions, chilli, a pinch of salt, and white pepper on top of the beef.
After 1-2 minutes, start tossing the mixture. Add the diced mushrooms and water chestnuts. Stir in the black pepper, dark soy sauce, light soy sauce, oyster sauce, and about 30 ml of the reserved mushroom stock.
Add the cornflour slurry and stir until the sauce thickens slightly. Add MSG. Mix everything until well combined.
Serve the beef mixture next to large leaves of baby gem or iceberg lettuce. Garnish with the green parts of the spring onions.