A note about the fish: I asked my brother a few days ago for good resources on finding sustainably-caught fish and he and my dad suggested these websites, from the Monterrey Bay Aquarium and NOAA's National Marine Fisheries Services. Based on their recommendations, I went with wild-caught Pacific (Alaskan) halibut, but cod works as well.
4 5-to 6-ounce fish fillets (such as halibut or cod; each about 1 inch thick
3 tablespoons tarragon leaves
1 pound slender asparagus spears, trimmed, cut into 1 1/2-inch pieces
1 pound carrots, sliced into 1 1/2-inch pieces
1/2 red onion, thinly sliced
juice from 1 lime
Preheat oven to 400°F. Place parchment squares on work surface. Drizzle ~2 tsps of olive oil in the center of each parchment square; top each one with 1 fish fillet. Sprinkle fish with salt and pepper; divide tarragon, asparagus, carrots and red onion evenly among 4 fillets, then drizzle with another ~1-2 tsps olive oil and lime juice. Fold parchment over fish and asparagus, folding and crimping edges tightly to seal and enclose filling completely. Place on 2 rimmed baking sheets (or a glass baking dish or two), spacing apart.
Here's a photo of the set-up, pre-baked.
Bake fish packets 17-25 minutes. Slide packets onto plates and serve. I ate it with steamed brown rice - delicious! Here's the final product: