Preheat oven to 400°. Place salmon on a large piece of parchment paper or heavy-duty aluminum foil with at least 2 inches to spare on each end and enough room on the sides to fold over entire fish.

In a medium bowl, combine capers, onion, tomato, and garlic. Spoon over salmon. Drizzle with lemon juice and sprinkle with salt and pepper. Top with asparagus spears and rosemary sprigs. Drizzle with olive oil.

Wrap paper or foil securely around salmon with edges at the top (secure paper with metal paper clips if necessary). Place on a baking sheet and cook for 20–25 minutes or until cooked through. Be careful when opening the package; clips and escaping steam will be quite hot. Serve with lemon wedges.