Step 1: Preheat your oven to 400°F (200°C). Prepare four large (about 12x18 inch) sheets of heavy-duty aluminum foil. If you only have standard foil, use two sheets per packet for extra durability to prevent tearing and leakage.
Step 2: In a small microwave-safe bowl, combine the melted butter, minced garlic, lemon juice, dried dill, salt, and black pepper. Stir well to combine.
Step 3: Place one salmon fillet in the center of each foil sheet. Arrange a portion of the trimmed asparagus spears next to each salmon fillet.
Step 4: Drizzle the garlic butter sauce evenly over the salmon and asparagus. Ensure both the fish and vegetables are nicely coated.
Step 5: Bring the long sides of the foil together over the salmon and asparagus, then fold them down tightly to create a sealed packet. Fold in the short ends to completely seal each packet, leaving a little room for steam to circulate inside. Make sure the packets are sealed well to trap all that delicious flavor.
Step 6: Place the sealed foil packets on a baking sheet. Bake for 12-18 minutes, depending on the thickness of your salmon fillets. Salmon is cooked when it flakes easily with a fork and reaches an internal temperature of 145°F (63°C). Thinner fillets will cook faster, so start checking around 12 minutes.
Step 7: Carefully remove the baking sheet from the oven. Be cautious when opening the foil packets as steam will escape. Use tongs or a fork to gently open each packet.
Step 8: Garnish with fresh chopped parsley or an extra squeeze of fresh lemon juice, if desired. Serve immediately, enjoying the tender salmon and crisp-tender asparagus infused with the glorious garlic butter sauce straight from the foil.