Position a rack in the center of your oven and heat to 375°F. Tear off a large sheet of heavy-duty foil and lay it on a rimmed baking sheet — regular foil works too, just use a double layer to prevent tears.
In a small bowl, whisk together the melted butter, lemon juice, lemon zest, minced garlic, dried dill, and generous pinches of salt and pepper. The mixture should smell bright and herbaceous — taste it and adjust seasoning as needed.
Place the salmon in the center of your foil, then bring up the sides to create walls around the fish. Pour the butter mixture evenly over the salmon, making sure it pools around the fillets. Bring the foil edges together and crimp tightly to create a sealed packet — no steam should be able to escape.
Slide the baking sheet into the oven and bake until the salmon flakes easily with a fork and reaches 145°F at the thickest part. Thinner fillets will be done closer to 15 minutes, while thicker center-cut pieces may need the full 20.
⏱ 20 min
Carefully open the foil packet — watch out for the burst of hot steam. Transfer the salmon to plates and spoon over any accumulated buttery juices. Scatter fresh dill on top if using, and serve right away while the fish is still steaming.