Grease perforated base of steamer pan with butter; place cod (thawed, if frozen) on buttered surface.
Pour enough boiling water into bottom pan to fill 1/2 full.
Place perforated pan over bottom pan; steam about 15 minutes, until cod flakes easily.
Remove cod; cool.
Remove skin and bones; season with salt.
Place fillets on serving platter; spoon mayonnaise around fillets.
Garnish wtih lemon slices and cloves.