Heat oven to 425ºF. Line a rimmed baking pan with nonstick foil or parchment.
Zest and juice lemon. In a small saucepan, bring lemon juice and sugar to a simmer. Simmer until reduced to about 1 tablespoon, about 4 minutes. Remove from heat and add the lemon zest, wine, if using, and 6 tablespoons of the butter. Stir until butter is melted and set aside.
Mix together the salt, garlic powder and pepper. Cut halibut into 4 pieces and sprinkle all over with the salt mixture (you may have some leftover). Place on one half of the lined pan. Put ½ tablespoon of butter on top of each piece of fish.
Add asparagus to the sheet pan, season with remaining salt mixture and drizzle with olive oil. Place in the oven to roast. Cook until fish is just opaque (no longer transparent), and golden brown in parts, about 9 minutes. Turn on broiler and place pan under the broiler for 1 to 2 minutes to caramelize the top of the fish.
On plates, drizzle some sauce over the fish and asparagus. Garnish with parsley and serve with lemon wedges.