Preheat oven to 450°.
Snap off tough ends of asparagus.
Coat a nonstick baking sheet with cooking spray. Arrange asparagus in a single layer on baking sheet; lightly coat asparagus with cooking spray. Sprinkle asparagus with 1/4 teaspoon salt and 1/4 teaspoon pepper. Bake at 450° for 8 to 10 minutes or until asparagus is crisp-tender, shaking pan occasionally.
Combine remaining 1/2 teaspoon salt, 1/4 teaspoon pepper, buttermilk, and remaining 5 ingredients in a small bowl; stir with a whisk. Cover and chill. Serve asparagus warm or at room temperature with mustard mixture.