Make the soy-wasabi marinade: whisk together dry mustard and 2 teaspoons hot water to make a paste. Transfer mustard mixture to food processor; Add wasabi, ¼ cup soy sauce, and garlic powder and blend.
Place salmon fillets in a shallow baking dish and cover with marinade. Cover and refrigerate for 30 minutes.
Preheat the broiler to high. Place salmon on a lined baking sheet and broil until salmon is just cooked, about six to eight minutes.
In a large sauté pan, heat ginger, garlic, 2 tablespoons soy sauce, and 1 teaspoon each of peanut and sesame oil over medium heat. Add onions and cook until translucent, about five minutes. Remove onions with a slotted spoon and set aside.
Place the asparagus, all facing the same direction, in the pan and sauté in the sauce until cooked but not soft, about three to five minutes, turning with tongs as they cook. Remove from heat.
Plate salmon along with the asparagus and onion mixture. Top with orange zest and chopped cashews.