Boil the potatoes in salted water until soft. Drain, keeping about 2/3 cup (160ml) of the cooking water. Mash the potatoes and beat in the extra-virgin olive oil or olive oil and sunflower oil. Season with salt and pepper and stir in just enough of the cooking water to give a soft, slightly moist mash. Then stir in the olives, tomatoes, chile (if using), and herbs and mix well.
Warm the 2 tbsp oil in a nonstick skillet over medium heat. Add the fillets, skin-side down, and cook for about 3 minutes, then turn them over and cook for 30 seconds, until cooked through.
Serve the fish with the mashed potatoes.
Notes
OlivadeFor an olive paste to serve with the fish, blend 2/3 cup (110g) pitted black olives with a crushed garlic clove, 7 tbsp (100ml) extra-virgin olive oil, and 1 tbsp sherry vinegar.