After a long, slow roasting in the oven, salmon becomes even more succulent.
Preheat oven to 250 degrees, with rack in upper third. Place salmon on a parchment-lined baking sheet. Sprinkle both sides of each fillet with 1/4 teaspoon salt; season evenly with pepper. Bake until opaque but still bright pink in the middle, 25 to 30 minutes. Remove from oven.
Meanwhile, make relish: In a small bowl, stir together shallot, capers, lemon zest and juice, and remaining 1/4 teaspoon salt; season with pepper. Add oil, parsley, and mixed herbs; toss to combine.
Place asparagus in a steamer basket set over a pot of simmering water; steam until spears are crisp-tender and bright green, 3 to 4 minutes. Divide asparagus among serving plates. Place a salmon fillet on each plate next to asparagus, and spoon relish on top of fish.