To make the marinade, place olive oil and 1 sprig of rosemary into a resealable plastic bag. Add the fish steaks, seal the bag and turn gently to evenly coat the fish. Refrigerate for 1 hour.
Preheat oven to 230 degrees C.
Pour 1/4 cup olive oil into an oven-safe frypan. Stir in the onion, cherry tomatoes, corn kernels, garlic, 1 sprig of rosemary and sea salt.
Place frypan in preheated oven and cook until vegetables are soft, 20 to 30 minutes.
Preheat bbq for high heat. Lightly oil grate, and set 15cm from the heat.
Place the fish on the preheated bbq and season with salt and pepper to taste. Cook, turning once, until fish is opaque and flakes easily with a fork, about 5 minutes each side depending on thickness. Place the fish on a serving platter, and top with corn and tomato relish.