Season the mackerel with salt and pepper and set aside.
Heat the palm oil in a large pot over medium heat.
Add the sliced onions and fry until fragrant and browned.
Add the mackerel and fry on both sides.
Add the mixture from Part A and the remaining ingredients from Part B.
Cover and simmer on medium-high heat for 15-20 minutes.
Remove the mackerel and set aside.
Add the bitter leaf and amaranth; stir well to combine with the sauce. Continue to stir until the vegetables are wilted and reduced. Taste and adjust seasoning if desired.
Return the mackerel to the soup. Simmer uncovered for another 2-3 minutes.