Skin the trout fillet carefully, then season both sides with salt and black pepper. Set aside in the fridge while you prepare the filling. Melt the butter in a large saucepan over medium heat. Add the chopped onion and cook until soft and golden, about 10–12 minutes.

Add the chopped kale to the onions and cook until wilted, 2–3 minutes. Season with a pinch of salt and pepper.

Remove the skillet from heat. Stir in the dill, lemon zest, lemon juice, cream cheese, and crème fraîche until well combined and smooth. Allow the mixture to cool slightly.

Roll out the puff pastry to a size large enough to enclose the trout fillet with some excess around the edges. Lay the trout on one side of the pastry sheet. Spoon the creamy kale and onion mixture on top of the trout, spreading it evenly. Add the other roll of the pastry over the trout, then use a sharp knife to cut around the fish shape, leaving about a 2 cm border. Reserve any excess pastry for decoration if desired.

Press down the edges to seal, then crimp with a fork to ensure the filling stays inside. If using, brush the entire pastry with beaten egg yolk for a golden finish.

Place the Wellington on a baking sheet lined with parchment paper. Bake in a preheated oven at 200°C (390°F) for 20–30 minutes, or until the pastry is golden brown and crisp.

Allow the Wellington to rest for 5 minutes before slicing. Serve warm with your favourite sides!