Preheat the oven to gas 6, 200C, fan 180°C Line a baking sheet or griddle pan with foil. Make the gremolata by combining the parsley leaves, olives, capers, garlic, and lemon zest.
Put the fish fillets, skin-side down, on the foil and sprinkle a few teaspoons of the gremolata over each. Drizzle with the oil, season with black pepper, then roast for 10 minutes, or until the fish is cooked through and flakes when parted with a fork.
Meanwhile, steam the broccoli for 2 minutes, then add the asparagus. After 1 1/2 minutes, add the courgette; continue steaming everything for 1 1/2 minutes, until al dente.
Mix the remaining gremolata with the mayonnaise to make a tartare sauce. Serve the fish with the steamed veg, tartare sauce, and a lemon wedge.