Heat butter and olive oil in a pan over medium heat. Add onion and cook until soft. Add garlic and stir for 1 minute. Toss in spinach and cook until wilted. Remove from heat, mix in cream cheese, lemon juice, salt and pepper. Let it cool slightly.

Preheat your oven to 200°C (400°F). Line a baking tray with parchment paper. Roll the pastry sheet gently on a clean surface. Make sure it’s large enough to wrap the salmon.

Pat the salmon dry with paper towels. Sprinkle salt and black pepper. Spread a thin layer of Dijon mustard on top. This adds a subtle tangy flavor that pairs with the spinach.

Place half of the spinach filling in the center of the pastry. Put salmon on top. Cover with the remaining spinach mixture. Fold pastry over the salmon carefully and seal edges.

Brush the pastry with beaten egg. This gives a golden, shiny finish. You can also score light lines on top for decoration.

Place on the baking tray and bake 25–30 minutes. The pastry should turn golden and crisp. The salmon inside will be tender and cooked just right.

Remove from oven and let it rest 5 minutes. This keeps the filling from spilling when slicing. Serve warm for best taste.