Dice the potatoes into 1-2 cm cubes. Heat 15 ml of olive oil in a skillet over medium-high heat. Add the potato cubes and cook, stirring occasionally, until golden brown and tender, about 15-20 minutes. Season with salt and pepper during the last few minutes of cooking.

While potatoes cook, prepare the salsa. Dice the tomatoes, bell pepper, and red onion. Mince the garlic and finely chop the fresh parsley.

In a small bowl, combine the diced tomatoes, bell pepper, red onion, minced garlic, and chopped parsley. Drizzle with the remaining 5 ml of olive oil and squeeze in the juice from half a lemon. Season with salt and pepper. Stir gently and set aside.

Pat the mackerel fillet dry with paper towels. Score the skin a few times with a sharp knife. Season both sides with salt and pepper.

Heat a separate non-stick skillet over medium-high heat. Add a splash of olive oil (or use the oil from cooking potatoes if enough). Place the mackerel fillet skin-side down in the hot skillet.