Instructions

1. Slice 2 of the onions and set aside. Heat 2 tbsp. of the olive oil in a medium saucepan over medium heat. Add sliced onions and a pinch of salt and cook, stirring often, until very soft and golden, 20-30 minutes. Add stock and reduce by half, about 4 minutes. Add cream, bring just to a boil, then reduce heat to medium-low and simmer until pale golden and thickened, 10-15 minutes. Puree onion mixture in a blender until smooth. Transfer to a medium bowl and set aside. Meanwhile, dice the remaining onion. Heat the remaining 1 tbsp. olive oil in a small skillet over medium heat, add onions, and cook, stirring often, until soft, about 5 minutes. Season to taste with salt and pepper, then add to bowl with puree and set sauce aside.
2. Preheat oven to 425º. Roll out puff pastry on a lightly floured surface to 1⁄4" thickness. Invert a 4-cup deep oval baking dish lightly onto pastry. Using the tip of a small sharp knife, cut out an oval 2" wider than the dish, discarding trimmings. Remove dish from pastry and set aside. Mix together yolk and 1 tsp. water in a small bowl and set egg wash aside.
3. Season fish to taste with salt and pepper. Put half of the sauce into the baking dish and lay fish, skin side down, on top. Cover with the remaining suace and drizzle with truffle oil. Drape puff pastry oval over dish and brush with egg wash. Bake until deep golden brown and puffed, about 25 minutes. To serve, cut puff pastry in half crosswise, lift off each piece and set on the side of two plates, then divide fish and sauce between both plates.