Preheat the oven to 220°C (top and bottom heat). Cut the unpeeled potatoes into 1 cm thick slices and place them on a baking tray lined with baking paper. Score the slices in a diamond pattern to a depth of approx. 5 mm. Sprinkle the potatoes with salt, pepper and rosemary, and drizzle with 3 tbsp. olive oil. Bake on the middle shelf for approx. 30 minutes until crispy. Heat the spinach with 2 tbsp. water in a saucepan over a low heat, stirring occasionally.
Halve the tomatoes, cut the fish into four fillets and season with salt and pepper. Heat 3 tbsp. oil in a large frying pan and fry the fish fillets on both sides. During the last few minutes, add the rosemary, tomatoes and olives and fry together.