Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Pat the salmon fillet completely dry with a paper towel and place it on one side of the prepared baking sheet.
In a small mixing bowl, whisk together the Dijon mustard, olive oil, lemon juice, garlic powder, sea salt, and black pepper until smooth.
Finely chop the fresh parsley and dill, then stir them into the mustard mixture to create a thick herb paste.
Spread the herb paste evenly over the top of the salmon fillet, pressing it down gently so it adheres to the fish.
Arrange the asparagus spears on the empty side of the baking sheet in a single layer, ensuring they are not overcrowded.
Bake for 12 to 15 minutes, depending on the thickness of the fillet, until the salmon flakes easily with a fork and the asparagus is tender-crisp.
Remove from the oven and serve immediately, optionally finishing with an extra squeeze of fresh lemon juice.