Fold four 20-inch-long parchment sheets in half lengthwise. Unfold one sheet at a time. Mound one quarter of the asparagus on one side, parallel to crease. Top with 1 fillet; season. Top with 2 dill sprigs, 3 lemon slices and 2 pats butter; season. Fold parchment over; fold up edges to seal. Bake pouches on sheet at 375 degrees until fish flakes, 18 minutes.