Mix salad dressing and lemon juice, pour half of the mixture into a large resealable bag. Add fish; seal bag, turn several times to coat. Refrigerate 15 minutes. Refrigerate other halp of dressing for later.
Remove fish from marinade, discard.
Heat barbecue to medium heat. Cut each lemon into 4 slices, place 8 slices on barbecue grate in sets of 2. Place fish fillets over lemon slices, brush with some of the reserved dressing mixture. Cover lid.
Grill fish 5 minutes; turn over, add remaining lemon slices and the asparagus to barbecue. Grill 5 minutes, brushing asparagus with dressing mixture, until tender-crisp.
Remove fish, lemon slices and asparagus from girll, discard lemon slices under the fish. Serve remaining lemon slices with the gish and asparagus.