Cut 4 pieces of aluminum foil to 12" each. Place about 1/4 of the asparagus (about 6-7 stems) and 1/4 of the onion slices on each piece of foil. Place a salmon fillet on each pile.
Drizzle 1 tbsp of olive oil and sprinkle salt and pepper on each. Move the salmon and asparagus around to make sure it's evenly coated.
Squeeze about 3 slices of lemon on top of each salmon and lay them on top. Place a sprig of dill on each.
Bring the long sides of the foil up to the middle and fold the edges together. Roll up each of the sides and place them on a cookie sheet. Bake for 15-18 minutes or until salmon is flaky.