Combine onions, habanero pepper, lime juice, vinegar, garlic and bay leaf in bowl.
Season fish with salt and pepper. Pour onion mixture over the fish and marinate in refrigerator for approximately 1 hour.
Preheat grill until hot.
Remove fish from marinade, scraping off any bits of the marinade. Reserve marinade.
Brush fish with oil to prevent it from sticking to the grill.
Arrange fish on the grill and cook for 3 minutes on each side. Remove from grill.
Cook reserved marinade with onions over medium-high heat in a saucepan. Reduce heat to medium-low; cover and cook for 10 to 15 minutes (stirring occasionally). Add a little water if too dry. Adjust seasonings.
Add the grilled fish and olives into the sauce and cook through.
Serve with a side of fermented cassava and Moyo relish.
Combine all ingredients in a bowl and stir well. Refrigerate until ready to serve. Relish may be made a day ahead.