Mix the vinaigrette and chill. Cut the tough ends (about 1.5 inches) off the asparagus. Use a peeler to shave the asparagus, discarding the first shave. When you are ready to serve the salad, drizzle just enough vinaigrette to coat the asparagus. Immediately plate the asparagus with one portion each of the black cod, grapefruit segments, and some of the pumpkin seeds. Garnish with tarragon leaves.