Instructions

Make the crab butter

• Bring 1 inch of water to a boil in a 4-quart saucepan fitted with a steamer basket. If using live crabs, hold them with tongs and rinse under cold water to wash off any sand or seaweed. Put the crabs in the saucepan and steam, covered, until the shells are bright red, about 10 minutes.
• Remove the crabs from the steamer and let cool. Remove the steamer from the saucepan and wipe out the pan.
• Return the crabs to the saucepan, and with a meat mallet or heavy wooden spoon break up the shells, legs, and claws until some of the meat is exposed. Add 1/2 lb. of the butter, the paprika, turmeric, 1/4 tsp. salt, and a few grinds of white pepper or a pinch of cayenne. Over low heat, melt the butter and bring to a simmer. Cook, occasionally crushing the shells more, until the butter is infused with the crab flavor, about 30 minutes.
• Remove from the heat and let sit for 10 minutes. Strain the butter through a fine-mesh strainer into a medium heatproof bowl, pressing on the solids to extract all the liquid. Set the bowl over a larger bowl filled with ice water.
• Whisk the butter until it starts to firm up, about 2 minutes. Transfer to a mini food processor and add the remaining 2-1/2 Tbs. butter, cut into 1/2-inch cubes. Process until smooth and well blended.
• Scrape the butter into a small bowl, cover with plastic, and refrigerate until firmly set, about 2 hours. You can make the butter up to 5 days ahead.

Make the papillote

• Position racks in the upper and lower thirds of the oven and preheat oven to 400°F.
• Melt 2 Tbs. of the crab butter in a 12-inch skillet over medium heat. Add the onion and fennel and cook until they begin to caramelize, about 15 minutes. Add the wine and bring to a simmer, scraping up any browned bits on the bottom of the pan, about 2 minutes. Turn the heat up to medium-high and fold the spinach into the onion mixture. Cook, stirring occasionally, until the spinach is completely wilted, about 3 minutes. Season to taste with salt and pepper and set aside.
• Rinse the trout fillets under cool water. Pat dry with paper towels and place on a baking sheet. Season both sides generously with salt and pepper, and sprinkle the tops of the fillets equally with the tarragon and the lemon zest.
• Pat the trout fillets dry with paper towels and season generously with salt and pepper. Sprinkle the tops of the fillets evenly with the tarragon and lemon zest.
• Cut four 15×24-inch sheets of parchment. Fold each sheet in half, forming a 15×12-inch rectangle. With a pencil, draw a half-heart on each, centering it on the folded edge. Cut out the hearts. (This shape is easier to seal than a rectangle.)
• Unfold one of the parchment hearts and arrange on a work surface. Coat the inside with vegetable oil, leaving a 1-inch border.
• Place 1/2 cup of the spinach mixture (without liquid) on one half of the heart near the fold. Place one fillet on top of the spinach. Spread 1 to 2 Tbs. of crab butter on top of the fillet (reserve remaining crab butter for another use). Top with 1/2 cup of grape tomatoes. Season with a sprinkling of salt and a few grinds of pepper.
• Fold the other half of the parchment over and line up the edges. Starting at the top of the heart, fold over about 1/2 inch of the edge, pressing down to make a crisp crease. Continue working your way around the edge of the packet, making overlapping folds (like pleats), always pressing firmly and creasing the edge so the folds hold. Twist the tip of the heart to finish. If necessary, make a second fold anyplace that doesn’t appear tightly sealed. Repeat with remaining the parchment and ingredients.
• Transfer the packets to two large rimmed baking sheets and bake, until the packets are puffed and fragrant, 10 to 12 minutes.
• Place a packet on each plate and slice or tear open the top, folding back the edges of the parchment. Serve immediately, garnished with lemon wedges.