Fill a large pot or high-rimmed skillet with about 3 inches of water and bring to a boil over high heat. Add a generous pinch of salt to the boiling water and then add the asparagus. Reduce the heat to medium and cook the asparagus until crisp-tender, about 3-5 minutes, depending on the thickness and freshness of the asparagus. Drain, rinse under cold water (to stop the cooking and preserve the asparagus' bright green color) and transfer to a paper towel-lined plate. Set aside.
While the water is coming to a boil and the asparagus is cooking, make the dressing. In a medium bowl, place the sliced spring onions, fennel pollen, salt, pepper and white wine vinegar, and toss to combine. Let the onions macerate for 10 minutes (this will tame the sharpness of the onions and allow the fennel pollen to infuse into the vinegar).
While the onions are macerating, cut the hard-boiled egg in half and remove the yolk. Finely chop the whites and set the yolk aside for garnish. To finish the dressing, add the Dijon mustard and the extra virgin olive oil and mix with a fork until the dressing is emulsified. Add the chopped egg whites and parsley and gently stir to combine. Taste and adjust seasonings, if necessary.
To serve, arrange the blanched asparagus on a platter and spoon the vinaigrette over them. Crumble the reserved egg yolk over the top and finish with an additional drizzle of good extra virgin olive oil and a sprinkling of fleur de sel.