For the herb vinaigrette:
Add the dry mustard, garlic salt, ¼ teaspoon of the kosher salt and black pepper to a pint size jar.
Add the lemon zest and juice along with the chopped parsley, tarragon, scallions and capers.
Pour in the olive oil and champagne vinegar. Tightly close the lid and shake well to mix. Check for seasoning, adding the remaining kosher salt if needed. 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, trim if desired. Rinse and pat dry.
Spread the asparagus out in a single layer on a rimmed baking sheet, lined with a Silpat liner or foil. Drizzle the asparagus with olive oil, using enough toss to coat the spears. Sprinkle with kosher salt and black pepper.
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 spear. Cool to room temperature or chill until you're ready to serve.
For serving:
Arrange the roasted asparagus on a platter. Drizzle the herb vinaigrette over the asparagus and sprinkle extra capers over the top. You will have extra vinaigrette.
Drizzle the herb vinaigrette over the asparagus and sprinkle the remaining capers over the top.
Serve warm, chilled or at room temperature.