Place the chopped red onion in a small bowl, season with salt and cover with the vinegar, set aside.
Place the eggs in a pot of boiling water and simmer for 5 ½ minutes. Once cooked, place them under cold running water until cool and set aside. In the same pot, place the asparagus and simmer for 4-5 minutes, then drain and cool under cold running water and set aside.
Drain the vinegar from the onion into a small bowl and mix in the Dijon mustard and olive oil to make the salad dressing.
Divide the rocket, asparagus and onion between two plates. Peel and half the egg, place on top of the salad, drizzle with the salad dressing and season with ground black pepper to serve.