Stuffed Flounder with Crab Stuffing

Steps

1. Preheat the oven to 400°F (200°C), rack in the middle. Butter a 9 x 13-inch (23 x 33 cm) baking dish with a small knob taken from the 80 g of butter. Take the fish out of the fridge 10 minutes before using it.
2. Cut the celery into tiny 1/8-inch (3 mm) dice. Melt 20 g of the butter in a small skillet over medium heat and sweat the celery for 4 minutes without browning, until translucent but still slightly crisp. Transfer it to a mixing bowl and let it cool.
3. Drain the crab meat, squeeze it gently in your hands to remove excess liquid, and run it through your fingers to pick out any bits of shell. Finely chop the parsley. Zest one lemon, juice it, and cut the second lemon into wedges.
4. In the bowl with the celery, whisk the egg with the mayonnaise, mustard, paprika, cayenne, lemon zest, 1 tablespoon of the lemon juice, two thirds of the parsley, and the pepper. Add 35 g of the breadcrumbs, then the crab, and fold with a fork without mashing: you want visible chunks of crab. Salt only lightly, since crab is already salty. The stuffing should hold together when you press it.
5. Pat the flounder fillets dry with paper towels and lay them skinned side up (the duller side), wide end toward you. Season lightly with salt. If a fillet is very wide, cut it in half lengthwise along the center line.
6. Place a spoonful of stuffing on the wide end of each fillet, about one eighth of it, then roll toward the tip, snug but not squeezed. Set the rolls upright or seam side down in the dish, packed close so they support each other. Any stuffing peeking out on top will brown nicely.
7. Melt the rest of the butter with the grated garlic for 1 minute over low heat, then off the heat add 2 tablespoons of lemon juice and a pinch of salt. Spoon half of this lemon butter over the rolls, then scatter the remaining breadcrumbs on top.
8. Bake for 18 to 22 minutes, depending on thickness. The fish is done when the flesh is white and opaque and flakes with a fork; a thermometer pushed into the center of a roll, through the fish into the stuffing, reads 145°F (63°C). If the breadcrumbs are still pale, slide the dish under the broiler for 2 minutes, watching closely.
9. Let rest for 3 minutes, baste with the pan juices and the rest of the warm lemon butter, sprinkle with the remaining parsley, and serve with the lemon wedges.