Cut cauliflower florets in half. Mix water and mushroom liquid with cornflour, 1 tablespoon fish sauce and oyster sauce.
Pour 1 tablespoon of fish sauce over sliced meat and sprinkle pepper over. Turn meat and let stand 20 minutes.
In a pan, heat oil over high heat. Add garlic and onion and stir-fry until onion separates and softens. Add cauliflower and mushrooms. Cover, reduce heat and cook for 4 minutes. Add meat and cook until meat is cooked to your liking. Stir in cornflour mixture. Continue stirring until sauce thickens. Spoon onto a serving plate and garnish with coriander sprigs.
