Whisk together dill, lemon juice, oil, mustard, salt, sugar, pepper, and garlic; pour into a resealable plastic bag. Place fish in bag, seal securely, and chill at least 20 minutes. Take fish out of bag and throw away marinade.
Preheat grill or broiler.
Lay fish on a cooking spray coated grill rack and cook 4 minutes per side; fish will flake easily when finished.