Preheat oven to 400 degrees F. Cut four 12x16 inch sheets of heavy-duty foil and arrange on your counter. Lightly coat the center of each with cooking spray or a thin layer of oil.

In a small bowl, whisk together melted butter, minced garlic, lemon zest, half the lemon juice, dill, 1/2 teaspoon salt, and black pepper.

Divide asparagus, tomatoes, and onion evenly among the four foil sheets, piling vegetables in the center. Nestle a salmon fillet on top of each vegetable pile. Spoon the garlic butter evenly over the salmon and vegetables, letting some drip down onto the vegetables. Sprinkle remaining 1/4 teaspoon salt over the vegetables.

Bring the two long sides of foil together over the salmon and fold down tightly in 1-inch folds, leaving room for steam. Fold in the short ends twice to seal completely. The packets should look like little half-moon parcels with no gaps for steam to escape.

Arrange packets on a rimmed baking sheet and bake for 18-20 minutes, until salmon flakes easily with a fork and reaches 145 degrees F internally. The vegetables should be tender but not mushy.

Carefully open packets - hot steam will escape. Drizzle with remaining lemon juice and serve directly from the foil or slide onto plates. Spoon any accumulated juices over the top.