Preheat oven to 400°F (205°C).
Line a sheet pan with parchment paper.
Place asparagus, tomatoes, and red onion on one side of the pan.
Drizzle vegetables with olive oil and season with salt and pepper.
In a small bowl, combine olive oil, lemon juice, lemon zest, garlic, parsley, rosemary, salt, and pepper.
Place salmon on the remaining side of the pan and brush generously with the lemon herb mixture.
Arrange lemon slices over the salmon.
Roast for 18–20 minutes, or until salmon flakes easily with a fork and vegetables are tender.
Remove from oven and allow to rest for 3 minutes.
Garnish with additional parsley before serving.