Boil potatoes until tender, then let cool and chop into bite-sized pieces. Blanch asparagus in boiling water for 2-3 minutes, then transfer to a bowl of ice water to stop the cooking process. Drain and chop into bite-sized pieces. Whisk together olive oil, lemon juice, Dijon mustard, and salt and pepper to make a dressing. Toss the potatoes and asparagus with the dressing, then add chopped hard-boiled eggs, sliced scallions, and chopped parsley. Serve cold.
