How to Make It
Preheat the oven to 450°. Line a large rimmed baking sheet with 2 overlapping 3-foot-long sheets of heavy-duty aluminum foil (half of each sheet should overlap in the middle of the pan). Top the foil with two 3-foot-long sheets of parchment paper, overlapping them in the same way.
In a large saucepan, steam the artichokes over moderately low heat, covered, until the bottoms are tender when pierced, about 50 minutes. Transfer the artichokes to a plate and let cool. When the artichokes are cool enough to handle, remove all of the leaves and reserve them for another use. With a teaspoon, scoop out the hairy chokes. Cut the artichoke bottoms into 1/3-inch wedges.
In a large bowl, toss the artichokes with the cherry tomatoes, leeks, wine, basil, olive oil and marjoram. Season generously with salt and pepper.
Season the outside of the sea bass with salt and pepper and set the fish in the center of the parchment-lined foil. Season the sea bass cavity with salt and pepper. Put the rosemary sprig in the sea bass cavity and scatter the vegetable mixture all around the sea bass. Fold the parchment neatly over the fish, then close the foil by folding the overhanging ends into the center and crimping the sides tightly. Bake the fish on the bottom shelf of the oven for 1 hour and 10 minutes, then let the fish stand at room temperature for 5 minutes.
Carefully open the package. Using 2 forks, gently lift the fish from the bones onto plates. Spoon the vegetables and juices around the fish and serve.