Preheat a grill to medium-high.
Rub the lime zest and juice over the mackerel, then massage with olive oil.
Brush with a little butter and grill for 3-4 minutes, skin-side up. Brush with more butter, turn the fillets over and continue cooking for 2-3 minutes until cooked through (the flesh should be opaque at the thickest part).
While the mackerel is grilling, make the sauce. Mix the soy, chilli and ginger in a small bowl, then stir in the sugar.
Once the fish has cooked, transfer to a serving plate and drizzle the sauce.
Serving Suggestions: Serve the mackerel drizzled with the sauce and scattered with the grated ginger and on steamed baby bok choi. Also, serve with lime wedges for your better taste experience.