Chop the red onions. In a small bowl combine red onions and pesto and mix.
Preheat oven to 200ºC (400ºF). Cover a baking sheet with parchment paper
Place the halibut on the parchment paper and spread a tablespoon of pesto on each filet. Bake 10 minutes. Sauté hazelnuts in the butter and add a few drops of lime juice. Place them on the fish fillets. Serve immediately with a green vegetable and rice.