For the herb vinaigrette:
Combine the dry mustard with the garlic salt and ½ teaspoon of the black pepper.
Add the lemon zest, parsley, tarragon, scallions, and capers.
Whisk in 6 tablespoons of the olive oil and the champagne vinegar. Cover and refrigerate for at least one hour.
For the roasted asparagus:
Preheat oven to 400-degrees.
Snap the tough ends off the bottom of the asparagus. Rinse and pat dry.
Spread the asparagus out in a single layer on a rimmed baking sheet. (Lining the baking sheet with heavy-duty foil makes for easy cleanup.)
Drizzle the remaining olive oil over the asparagus, toss to coat well.
Sprinkle with the kosher salt and remaining ½ teaspoon black pepper.
Roast in a 400-degree oven for 15 minutes or if the stalks are thin, roast for about 10 minutes – just until the tip of a sharp knife easily pierces the stem. Cool or chill until you're ready to serve.
For serving:
Arrange the roasted asparagus on a platter. Squeeze the juice from half of a lemon over the asparagus. Top with the chopped hard-boiled eggs.
Drizzle the herb vinaigrette over the asparagus and sprinkle the remaining capers over the top.
Serve chilled or at room temperature.