Defrost the mackerel in the refrigerator for several hours or overnight.
Cut the lemon into roughly 5mm thick circles (3 circles per mackerel).
If using whole mackerel with head and tail: remove the head and tail, gut the fish, rinse with cold water and pat dry. Cut the mackerel lengthwise down the center and open it flat like a book.
When the charcoal is hot (medium-high heat), place the opened mackerel skin-side down on the grill rack. Sprinkle 1 pinch of salt on the flesh and lay the lemon circles inside the fish. Secure the fish in the rack so it does not shift.
Grill for approximately 15 minutes, flipping the grill rack over halfway through, until the flesh is opaque and cooked through.