If using frozen bao buns, steam according to packet instructions (usually 6-8 minutes). Keep warm.
Heat oil in a pan over medium heat. Add chopped spring onions. Sauté for 2-3 mins until softened.
Add the slow-braised mushrooms and water. Slowly 'prize open' the mushrooms using spatula. Cook for around 5 minutes, stirring occasionally, until heated through.
Add garlic paste, ginger paste and hoisin sauce. Stir well. Cook for a further 2-3 mins. Remove from heat.
Carefully open each steamed bao bun. Spoon in a generous portion of the mushroom mixture.
Add fresh coriander leaves, grated salad, sriracha mayo and a sprinkle of black sesame seeds (optional).