Preheat oven to 180C. For pipérade, toss ingredients in a bowl, season to taste, then scatter over a baking tray with a lip and roast without stirring until golden and tender (45 minutes to 1 hour). Keep warm.
For parsley crust, process crumbs, parsley and garlic in a food processor to combine, add oil, pulse to combine, season to taste and press onto one side of fish.
Heat oil in a large non-stick frying pan over high heat and cook fish crust-side up for 1 minute. Add butter to pan then transfer pan to oven and roast until fish is cooked and crust is crisp (8-10 minutes). Set aside to rest in pan for 1-2 minutes, then serve hot with pipérade and lemon wedges for squeezing.