Combine asparagus, 2 teaspoons olive oil, salt, and garlic in a large bowl, tossing well to coat. Arrange asparagus mixture in a single layer on a baking sheet. Bake for 10 minutes, or until crisp-tender.
Combine remaining 2 teaspoons oil, lemon zest, juice, mustard, and pepper in a small bowl, stirring with a whisk. Arrange asparagus on a platter; drizzle juice mixture over asparagus. Sprinkle with parsley if desired.