Pat the salmon dry with paper towels, cut into thirds, season both sides with salt and pepper, and let sit for 5 minutes. Coat with flour.

Slice the onion thinly.

Heat half the butter (10g) in a pan, cook the salmon on medium heat until browned on both sides, and remove. Add the remaining butter (10g), sauté the onion until soft.

Add flour to the pan, stir until no raw flour remains. Add the remaining sauce ingredients, stirring until thickened. Let cool.

Dust a board with flour, layer two puff pastry sheets, and roll out slightly larger. Line a buttered baking dish with the pastry, trimming excess and pressing edges. Chill in the fridge for 30 minutes.

Poke shallow holes in the pastry base with a fork, line with parchment paper, and add weights. Bake at 200°C for 20 minutes. Remove weights, press down puffed areas, and bake for another 5 minutes.