Preheat oven to 375°. Using a sauté pan, cook the fennel and onion in virgin olive oil until translucent; add the clam juice and reserve.
Place the sea bass in the middle of the parchment paper. Cover with clam juice, fennel, tomatoes and a drizzle of olive oil. Bake for 4 to 6 minutes until the paper is filled with steam and it has obtained a nice brown color. Open the papillote and serve in a dish, sprinkle the fennel and chopped cilantro on top for garnish before serving.