Step 1

Position a rack in the middle of the oven and preheat to 425 degrees.

Step 2

Finely zest the lemon into a small bowl, then cut the lemon in half. Juice one half of the lemon into another small bowl; cut the other half into wedges. To the bowl with the zest, add the olives, garlic, thyme and 1/4 teaspoon of the pepper, and stir to combine.

Step 3

Arrange four 12-by-18-inch sheets of parchment paper on a flat work surface. (If you don’t have enough counter space, you may need to work in batches.) Fold each sheet in half lengthwise; and then open flat. Evenly divide the haricots verts on one half of each piece of parchment, near the fold. Arrange the tomatoes on top, then place a piece of fish on top of that. Sprinkle with the salt and the remaining 1/4 teaspoon of pepper, then top each piece of fish with a quarter of the olive mixture, distributing it evenly. Lay the onion slices on top of that, then drizzle each piece with 1 1/2 teaspoons of the olive oil and 1 teaspoon of the reserved lemon juice.

Step 4

Fold the empty parchment half over the top of the fillets and roll the edges, crimping to seal tightly. The paper can touch the fish, but should not be pressed against it, leaving the packets room to expand.

Step 5

Place the packets on a large, rimmed baking sheet and bake for 12 minutes, or until the fish is cooked through and beans are tender-crisp.

Step 6

Open each packet (be careful of the steam) and serve on plates, either directly in the packets or with the contents of each transferred to the plates, garnished with the parsley and lemon wedges, for squeezing.