Preheat and Prep: Set your oven to 410°F (210°C). Line a large baking sheet with parchment paper for easy cleanup.
Position Salmon: Arrange the salmon fillets on one side of the prepared baking sheet.
Mix Marinade: In a small bowl, whisk together the olive oil, lemon zest, lemon juice, minced garlic, dried thyme, dried dill, optional red pepper flakes, salt, and black pepper until well combined.
Coat Salmon: Spoon or brush about two-thirds of the citrus herb marinade evenly over the tops and sides of the salmon fillets.
Add Asparagus: Place the trimmed asparagus spears on the other side of the baking sheet. Drizzle the remaining one-third of the marinade over the asparagus and toss them gently to ensure they are lightly coated. Spread the asparagus in a single layer.
Roast: Transfer the baking sheet to the preheated oven. Bake for 11-13 minutes, or just until the salmon flakes easily with a fork and the asparagus is tender-crisp. Cooking time will vary slightly depending on the thickness of your salmon fillets.
Serve: Carefully remove the baking sheet from the oven. Serve the salmon and asparagus immediately. Store any leftovers in an airtight container in the refrigerator for up to 2-3 days.