Preheat broiler. Lightly oil a 17- by 12-inch shallow baking pan.
Whisk together miso, sugar, lemon juice, water, and pepper in a bowl.
Toss together asparagus, oil, and a pinch of salt in a large bowl.
Arrange fish, skin sides down, in baking pan and spread miso mixture evenly on top. Arrange asparagus in 1 layer around fish and broil 5 to 6 inches from heat until fish is just cooked through and asparagus is crisp-tender, 8 to 12 minutes.