01 - Preheat oven to 400°F. Line a large baking sheet with parchment paper or lightly grease it.
02 - In a small bowl, whisk together olive oil, lemon zest, lemon juice, garlic, oregano, salt, and pepper.
03 - Arrange the asparagus in a single layer on one side of the sheet pan. Place the cherry tomatoes and red onion (if using) on the other side.
04 - Drizzle half of the marinade over the vegetables and toss gently to coat.
05 - Nestle the salmon fillets among the vegetables. Spoon the remaining marinade over the salmon.
06 - Roast in the oven for 15–18 minutes, or until the salmon flakes easily with a fork and the asparagus is tender.
07 - Garnish with fresh parsley and an extra squeeze of lemon juice before serving.