Preheat the oven to 400˚F.
Lightly butter a 9x13 baking dish.
Arrange the asparagus in the baking dish; lightly coat the asparagus with cooking spray, season with salt and pepper, and toss to combine.
Arrange the salmon fillets over the asparagus and set aside.
In a small bowl combine butter, garlic, half of the lemon juice, lemon zest, maple syrup, rosemary, salt, and pepper; mash with a fork to create a paste.
Rub the paste all over the fish fillets. Then, top each fillet with lemon slices and add a few fresh rosemary leaves, if using.
Bake for 17 to 20 minutes, or until the fish flakes easily and the asparagus is fork tender. For a light, crisp, and browned top, I suggest popping it under the broiler for the last 3 to 4 minutes. Watch it closely, and do not overcook it.
Remove from the oven. Squeeze lemon juice over the salmon, garnish with fresh parsley, and serve.