Preheat the oven to 180°C (356°F). Line a baking sheet with parchment paper. Rinse the asparagus and trim the tough ends by 1 inch. Cut the remaining asparagus into 2-inch (5 cm) lengths. Spread the asparagus on the baking sheet, arranging it to one side. Drizzle with vegetable oil and sprinkle evenly with salt. Toss well and set aside.
Rinse and pat the cod dry with paper towels. Place the fish on the empty side of the baking sheet, skin side down.
Microwave 3 tablespoons of butter for 20 seconds or until melted. Grate the garlic and ginger using a ceramic grater for a fine texture. In a bowl, combine the melted butter, white miso, sriracha, sugar, grated garlic, and ginger, stirring until smooth. Melt the remaining butter in the microwave and mix with panko in a separate bowl.
Spread one or two spoonfuls of the miso mixture evenly over the fish with the back of a spoon. Top with the buttered panko, pressing gently to help it stick and stay in place during baking.
Bake for 15 minutes or until the panko is golden brown and the fish is firm. If the fish is thick, extend the baking time to ensure it is fully cooked. If the panko topping isn't crisp enough, broil for an additional 3 minutes.
Once baked, remove from the oven, and toss the asparagus with the remaining miso mixture to enhance the flavor. Serve immediately.