Soak the mackerel in cool water for 30 minutes. Drain, cover with fresh water in a saucepan, and simmer for 10 minutes. Drain, cool slightly, remove large bones, and keep the pieces intact.
Boil the green bananas in 6 cups lightly salted water for 15 to 20 minutes until tender, then drain.
Make the Rundown:
Heat oil in a wide skillet. Cook onion, bell pepper, scallion, and garlic for 5 minutes. Add tomatoes and cook for 3 minutes.
Add coconut milk, thyme, Scotch bonnet, allspice, and black pepper. Simmer uncovered for 12 to 15 minutes until lightly reduced and able to coat a spoon.
Nestle in the mackerel and simmer gently for 5 to 7 minutes. Remove whole thyme stems and Scotch bonnet if used whole, then stir in lime juice.
Serve:
Spoon the mackerel and coconut sauce into shallow bowls and serve with boiled green banana.
Notes
Salt mackerel varies widely; taste after simmering before adding any extra salt.
Refrigerate for up to two days and reheat gently so the fish pieces stay intact.