Put the haddock fillets in the steamer. Cover tightly and steam for 8-10 minutes, depending on the thickness of the fillets: they should have turned opaque and be flaking slightly but is still moist. Remove onto warm plates and keep warm.
Place the pak choi in the steamer and steam for a minute or two.
Meanwhile make the dressing by whisking everything together in a small bowl.
Arrange the steamed haddock and pak choi on warmed plates and drizzle over the dressing to serve.