To make the sauce, combine the wasabi powder and water in a saucepan and let bloom for 5 minutes. Add 3/4 cup ponzu, the sake and grated ginger and bring to a boil over high heat. Reduce the heat to medium-low and simmer until the sauce is reduced by half, about 10 minutes.
Meanwhile, place the fish and asparagus in a large baking dish. Pour the remaining ponzu sauce and the oil over the fish and asparagus and turn to coat evenly. Let stand for 10 minutes.
In a small frying pan, toast the sesame seeds over medium heat, shaking the pan frequently, until lightly colored, 3 to 4 minutes. Immediately pour onto a plate to cool. Place a grill pan over medium heat until hot. Lift the asparagus from the marinade, place in the pan and cook, turning occasionally, until tender-crisp, about 3 minutes. Transfer the asparagus to a plate and cover with aluminum foil, shiny side down, to keep warm. Lift the fish from the marinade and place in the grill pan. Cook, turning once, until it turns opaque and just begins to flake, 2 to 3 minutes on each side.
To serve, divide the rice among 4 bowls. Top with a piece of fish and some asparagus. Pour 1/4 cup of the sauce over the fish in each bowl. Garnish each serving with the sesame oil, green onions, avocado, cucumber, pickled ginger and sesame seeds.
In a bowl, combine all the ingredients and stir until the sugar dissolves. Store in a covered container in the refrigerator; it will keep for up to 1 month.
Place the rice in a bowl. Add cold water to cover and wash the rice well by rubbing it between your hands, then drain. Repeat twice more. Pour the rice into a strainer to drain.
Place the rinsed rice and the 2 cups water in a 2 to 3-quart saucepan. Bring to a boil over high heat, reduce the heat to low, cover and cook for 20 minutes. Turn off the heat and let the rice stand for 10 minutes.
In a small pan, heat the vinegar, sugar, mirin and salt over medium heat until the sugar dissolves. Remove from the heat.
Turn out the cooked rice into a large wooden or plastic bowl, and pour the vinegar mixture evenly over the rice. Fold the liquid in with a wooden rice paddle. At the same time, fan the rice to bring out the luster of the grains. Continue fanning and folding until the rice absorbs all the liquid. Cover with a damp cloth and let stand until ready to use. It will hold at room temperature for up to 4 hours.