Preheat the oven to 200°C.
Heat half of the olive oil in a large frying pan over a medium heat. Sweat the shallot and garlic for 3-4 minutes, stirring occasionally, until softened. Add the cherry tomatoes and black olives, stir well and continue to cook for a further 3-4 minutes until the cherry tomatoes just start to break down. Remove from the heat and set to one side.
Heat the remaining olive oil in a large, heatproof frying pan. Season the halibut then sear in the olive oil until golden in colour on the outside. Transfer to the oven to finish cooking for 4-5 minutes until firm yet springy to the touch.
Meanwhile, reheat the sauce and stir in the chopped parsley. Season to taste with salt, black pepper and lemon juice. Spoon the sauce into the centre of 4 serving plates. Top with a halibut fillet and rest a sprig of parsley on top. Serve immediately.