Prepare the Glaze
Preheat the oven to 425°F. In a small saucepan, heat 2 tablespoons of neutral oil over medium heat. Add the minced garlic and cook for about 30 seconds, until fragrant. Stir in the soy sauce, mirin, sugar, black pepper, and ¼ teaspoon salt. Bring the mixture to a simmer, then reduce the heat to medium-low. Stir occasionally and cook for 10 to 15 minutes, allowing the sauce to reduce by about one-third and thicken. Remove from heat and stir in the freshly grated ginger. Set aside half of the sauce in a small bowl for later use.
Roast the Salmon
Line a sheet pan with aluminum foil and place the salmon fillet on the pan. Rub the salmon with the remaining tablespoon of oil and season with a pinch of salt. Coat the fish evenly on both sides with the prepared glaze.
Cook the Salmon
Place the salmon in the preheated oven and roast for 5 minutes. After 5 minutes, baste the salmon with any sauce that has dripped onto the sheet pan. Continue roasting the salmon for an additional 5 minutes, or until it reaches medium doneness, where it flakes easily with a fork. Remove from the oven and transfer to a large plate.
Assemble the Hand Rolls
Flake the roasted salmon into bite-sized pieces and drizzle with the reserved glaze. To assemble the hand rolls, place a quartered sheet of seaweed on a flat surface. If using, add a small portion of rice, then top with cucumber spears, avocado slices, and the flaked salmon. Wrap the seaweed around the filling, creating a neat roll. Optionally, serve with wasabi paste for added spice, and enjoy the combination of textures and flavors.