Roasted Asparagus with Poached Egg and Lemon-Mustard Sauce

INGREDIENTS

• 1 tablespoon unsalted butter
• 1 bunch thin asparagus
• 1/4 teaspoon plus 1/8 teaspoon kosher salt, divided
• 1/4 teaspoon plus 1/8 teaspoon freshly ground black pepper, divided
• 1/2 teaspoon finely chopped fresh tarragon leaves, plus additional for garnish
• 2 tablespoons apple cider vinegar
• 1/4 teaspoon lemon zest
• 3/4 cups heavy cream
• 1 tablespoon extra virgin olive oil
• 1/8 teaspoon Colman's mustard powder
• 4 eggs