Mix together the orange zest and juice, soy and vinegar. Line the top of a steamer with baking parchment and heat water in the base. Top the fish with a little of the ginger, add to the steamer and spoon over 2 tbsp of the dressing. Cover and steam for 5-6 mins until the fish flakes easily.
Meanwhile, heat the oil in a non-stick wok and stir-fry the spring onions, garlic, pepper and remaining ginger for 2 mins. Add the beansprouts and cook for 2 mins more. Pour any juices from the fish into the veg. Stir through the dressing, then divide between bowls, top with the fish and scatter over the sesame seeds.