Preheat the oven to 200ºC and scrub the potatoes and cut into 2-cm cubes. Put the cubes into a saucepan of boiling water and add salt. Cover, bring back to the boil and boil the potatoes for 10–15 minutes, until cooked.
Meanwhile, line a small baking dish with foil. Wipe the fish with paper towels and place the fillets in the baking dish. Spread the pesto evenly over each and season with the salt and pepper. Drizzle with olive oil and bake on the top shelf of the oven for 15–20 minutes or until the flesh flakes easily.
Drain the potatoes and return to the pan. Peel the garlic, crush it into the potatoes and mash. Stir in the butter and cream and reheat gently.
Serve the fish resting on a bed of potato and garnish with the basil.
Pesto is made from fresh basil, pine nuts, garlic, olive oil and Parmesan or Pecorino cheese, pounded together with a pestle and mortar. Red pesto also includes sun-dried tomatoes. When there is a glut of basil in the summer, pesto can be made quickly in a food processor and then stored in batches in the freezer for several months.