Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Wash the asparagus and trim off the tough, woody ends, then place the spears on one side of the prepared baking sheet.
Pat the salmon fillet dry with a paper towel to ensure a better roast and place it on the other side of the pan.
Drizzle the olive oil over both the salmon and the asparagus, using your hands or a brush to ensure the asparagus is evenly coated.
In a small bowl, combine the dried oregano, garlic powder, sea salt, and black pepper, then sprinkle the mixture evenly over the fish and vegetables.
Slice half of the lemon into thin rounds and place them on top of the salmon; squeeze the juice from the remaining half over the asparagus.
Bake for 12 to 15 minutes, depending on the thickness of the fillet, until the salmon is opaque and flakes easily with a fork.
Remove from the oven and garnish immediately with freshly chopped parsley before serving warm.