Preheat your oven to 450°F (190°C).

Scale and gut the mackerel fish. Rinse it thoroughly under cold water to remove any debris or blood. Use a sharp knife to make ½ to 1-inch diagonal cuts on both sides of the fish. Pat the fish dry with paper towels inside and out.

Slice the potatoes into thin, even slices. Season the sliced potatoes with salt and pepper to taste. Be mindful not to oversalt, as we will be seasoning the marinade.

Line a large baking sheet with aluminum foil. Drizzle a bit of oil on the aluminum foil to prevent the potatoes from sticking.

In a food processor or blender, combine the chopped red bell pepper, cilantro, garlic cloves, chopped ginger, olive oil, ground coriander, ground cumin, lemon juice, serrano pepper (if using), freshly ground black pepper, and salt.

Blend or process the ingredients until you have a smooth paste-like consistency. Adjust the salt according to your taste preferences.

Place the sliced potatoes on the greased baking sheet. Apply the marinade inside the scoring, in the guts, and on the skin on both sides of the mackerel, then place it on the sliced potatoes.

Spread the seasoning mixture generously over the surface of the mackerel, covering it evenly on both sides. You can use a brush or your hands to coat the fish thoroughly with the seasoning mixture.

Bake for approximately 20-25 minutes, or until the fish is cooked through and flakes easily with a fork. Cooking time may vary depending on the size and thickness of the mackerel.