Preheat oven to 425 F. Wash asparagus, snap off tough bottoms of spears, and arrange it on a baking sheet.
Mix water, soy sauce, and sesame oil in a small bowl. Add wasabi, starting with 1/4 teaspoon and increasing until the mixture is as spicy as you want it. Brush the wasabi mixture over the asparagus, being sure to coat all sides.
Roast until asparagus is tender but still crisp, about 10-15 minutes, turning once halfway through. Serve sprinkled with kosher salt, if desired.