Preparation Preheat oven to 275°F. Line a rimmed baking sheet with parchment paper. Toss asparagus with 2 Tbsp. oil, 1/2 tsp. salt, and 1/4 tsp. pepper on prepared sheet, then arrange in a single layer on one half of baking sheet. Bake asparagus 5 minutes. Meanwhile, spread each salmon fillet with 2 tsp. mustard. Sprinkle fillets with 1/4 tsp. salt and 1/4 tsp. pepper total. Toss panko with remaining 1 Tbsp. oil in a small bowl. Sprinkle panko mixture evenly onto salmon fillets, then dust with smoked paprika. After asparagus has baked for 5 minutes, remove baking sheet from oven. Place prepared salmon fillets skin side down on empty half of baking sheet. Return to oven and bake until salmon is firm but still pink in the center and asparagus is tender, 14–16 minutes more for medium rare salmon. Divide salmon and asparagus among four plates and top with tarragon. Cooks' NoteIf asparagus spears are thick, increase initial baking time to 10 minutes before adding the salmon fillets to the baking sheet. If you prefer your salmon more well done, increase cooking time by 5 minutes and check for doneness.