On a hot grill, add slices of yam, plantain, and whole dry salted fish. Grill until cooked through for 5 minutes on both sides, transfer the fish to a deep bowl, and allow the yam and plantain to finish cooking.
Pour hot water to cover it fully and let it soak for 30 minutes. Change the water repeatedly to reduce the salt content in the fish.
Drain the water and flake the fish off the bones and discard the bones leaving the head for the stew.
In a pan over medium heat, pour in palm oil, allow it to heat then add diced onions and fry for 3 minutes.
Add in the soaked fish heads and fry for 5 minutes on both sides.
Add in garlic paste, ginger paste, diced tomatoes, scotch bonnet pepper, and shrimp powder stir to combine, and let it simmer for 10 minutes.
Add in the fish flakes and allow to cook for another 3 minutes.
Serve the Koobi stew with roasted yam and plantain slices.