How to Make It
Preheat the oven to 350°. On a baking sheet, toss the tomatoes with the oil, vinegar and thyme. Arrange the tomatoes cut side up and season with salt and pepper. Bake the tomatoes for 45 minutes, until slightly dried.
In a large skillet, melt 1 tablespoon of the butter. Season the halibut with salt and pepper and add to the skillet. Cook over moderately high heat until lightly browned on one side, about 3 minutes; transfer to a plate.
Add the remaining 2 tablespoons of butter to the skillet and cook over moderate heat until it just starts to brown, about 2 minutes. Remove from the heat, stir in the lemon juice and season with salt and pepper.
Lay four 15-inch-long sheets of parchment paper on a work surface. Arrange 3 lemon slices in the center of each sheet and top with a halibut steak, seared side up. Scatter the tomatoes, olives and zucchini evenly around the fish, spoon on the butter sauce and sprinkle with the coriander. Fold the parchment over the fish and vegetables and fold the edge over itself in small pleats to seal.
Transfer the papillotes to a baking sheet. Bake for 15 minutes, until slightly puffed. Snip the parchment open with scissors and serve.