Crab Toasts with Asparagus

Name: Crab Toasts with Asparagus
Ingredients: 1/2 ounce dried porcini mushrooms (about 1/2 cup), finely ground; 1 large egg yolk; 1/4 cup plus 3 tablespoons hot water , divided; 2 1/2 teaspoons dry sherry or white wine; 2 teaspoons kosher salt , divided, plus more to taste; 1/2 teaspoon Dijon mustard; 3/4 cup extra-virgin olive oil; 1/4 cup rice vinegar; 1 tablespoon granulated sugar; 4 ounces asparagus , trimmed and thinly shaved on an angle (about 1 cup); 12 ounces fresh lump crabmeat , picked over; 3 tablespoons sour cream; Lemon zest plus 2 1/2 tablespoons fresh lemon juice, divided; 6 small sourdough bread slices (about 3/4 inch thick), toasted; Thinly sliced chives , for garnish