Preheat an outdoor grill for medium heat. Tear off 4 large squares of aluminum foil and spray with cooking spray.
Melt 1/3 of the butter in a skillet over medium heat. Toss asparagus, onion, dill, and seasoning blend with butter. Divide among the foil squares.
Top vegetables with swai fillets. Place small pats of remaining butter over each fish fillet. Sprinkle liberally with seasoning blend. Place lemon slices on top. Fold up foil into packets, sealing seams tightly but leaving room inside for steam to circulate.
Place packets directly on the grill and cook until swai flakes easily with a fork, 20 to 25 minutes. Open packets carefully and squeeze lemon juice over swai and vegetables.
