Preheat the oven to 180ºC, fan 160ºC, gas mark 4.
Toss together 1 tablespoon of the olive oil, the garlic and red onion wedges. Place in a roasting tin and lay the vine tomatoes on top.
Lay the fish over the vegetables and drizzle with the remaining olive oil. Season with sea salt and black pepper.
Bake for 30 minutes or until the fish is cooked through and serve scattered with fresh parsley and garnished with lemon wedges.