Wash and dry the mackerel with kitchen paper, then make 3 diagonal slits across each side of each fish, being careful not to cut right through.
Rub sesame oil over the inside and outside of the fish, then rub five spice all over, pushing plenty into the slits.
Place a few slices of lime and thinly sliced ginger inside the fish cavity and the rest on the base of the steamer, placing the fish on top. Steam for 8-10 minutes, depending on the size of fish.
Put the vegetables in another steamer tier on top and steam for 1 further 5 minutes.