Preheat the oven to 375 degrees F. Fold two 16-by-24-inch pieces of parchment paper in half like a book. Using kitchen shears or a chef’s knife, cut each into a large half-heart. When unfolded, a whole heart should appear.
Use a chef's knife or mandoline to very thinly slice the fennel bulb. Cut the leek in half lengthwise, then place on your cutting board flat side down. Slice lengthwise into thin strips (this is called julienne). Very thinly slice the bell pepper lengthwise.
To assemble the parchment paper packets, drizzle some olive oil on the bottom of each piece and rub it all over one side of each piece of parchment. Place the lemon slices to one side of the creased line down the middle of the parchment paper hearts. Top with the sliced fennel, leeks and bell pepper; season with a pinch of salt and pepper and a drizzle of olive oil. Drizzle the white wine over the veggies. Sprinkle the salmon with a good pinch of salt and few good grinds of pepper and top each with 1 tablespoon of butter. Then place on top of the veggies. To seal the packets, fold the other half of the heart over the fish. Starting at the pointy end or bottom of the heart, fold the edge of the parchment paper in small, overlapping triangles. This will help to create a tight seal and hold in all the steam. Transfer to a rimmed baking sheet. Bake until the vegetables are tender and the fish is just cooked through, 15 to 17 minutes.
Meanwhile, toss the fennel fronds, dill and parsley together in a small bowl until combined. Cut a slit in the top of each packet, then carefully open--there will be hot steam! Sprinkle with the fresh herb salad and serve.