Preheat the oven to 170°C fan (190°C conventional / 375°F).
Melt the butter in large frying pan over a medium heat. Tip in the red onion, season with a little salt and pepper, and cook gently for around 5 minutes until softened. Add the garlic and cook for 30 seconds, just until you can smell it.
Sprinkle in the flour and stir well until a paste comes together (the roux). Cook for 2–3 minutes then gradually pour in milk, whisking well to prevent lumps. Simmer gently for 5 minutes until thickened.
Stir in the double cream, shredded turkey, chopped ham and chopped parsley. Add the mustard and lemon juice.
Taste and adjust the seasoning – it will definitely want a good grind of black pepper.
Pour the filling into a pie dish (roughly 20–22 cm).
Then make the garlic butter: mix in a bowl the softened butter, garlic, parsley and some salt and pepper.
Add a few scoops of garlic butter on top of the pie filling.
Unroll the puff pastry and lay it over the top of the pie dish, trimming any excess around the edges. To seal it, press the edges down with a fork or your fingers. If you want to make the patchwork pattern, cut the sheet of puff pastry into small squares and lay them on top of the pie filling, slightly overlapping as you go.
Brush the top with beaten egg and a few sea salt flakes.
Bake in the preheated oven for 30–35 minutes, until the pastry is puffed and golden brown.
Allow the pie to rest for 5–10 minutes before serving.