Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper for easy cleanup.
Trim the woody ends off the asparagus spears and spread them out on one side of the prepared baking sheet.
Drizzle the asparagus with the olive oil and season with a pinch of sea salt and black pepper, tossing to coat evenly.
In a small mixing bowl, combine the Dijon mustard, finely chopped parsley, chopped dill, and garlic powder into a thick paste.
Place the salmon fillet on the other side of the baking sheet and season the top with the remaining salt and pepper.
Spread the herb-mustard mixture evenly over the top of the salmon fillet, pressing gently to ensure it adheres as a crust.
Roast in the center of the oven for 12 to 15 minutes, or until the salmon is opaque and flakes easily with a fork and the asparagus is tender.
Remove from the oven and serve immediately with a fresh squeeze of lemon over both the fish and the greens.