Martha prepares salmon fillets and asparagus with a piquant lemon-tarragon sauce.
Preheat oven to 400 degrees. Cut out four 12-by-17-inch pieces of parchment; fold each in half crosswise to form a crease, then open.
Divide asparagus and radishes evenly among 4 parchment pieces, arranging mixture on 1 side of each crease. Lay 1 salmon fillet on top of each pile. Season each with 1/8 teaspoon salt and some pepper.
Toss together onion, zest and juice, tarragon, 2 teaspoons oil, and 1/2 teaspoon salt; divide among salmon, spooning over tops. Fold parchment over ingredients; make overlapping pleats to seal.
Bake on 2 baking sheets (2 packets per sheet) for 11 to 12 minutes for medium-rare or 13 minutes for medium. Unwrap; drizzle fillets with remaining 2 teaspoons oil.