Wash and dry the spinach then roughly chop. Heat a splash of oil in a large pan then fry the garlic for a few seconds.
Add all of the spinach and allow to wilt and cook for a few minutes. Season with salt, pepper and a squeeze of lemon juice.
On a lightly floured surface, roll out the puff pastry.
Place the pastry on a parchment-line baking sheet. Season the salmon with salt and pepper then place on top of the pastry.
Add the spinach to the top of the salmon then cover with the pastry. Crimp the edges and gently score the top. Brush with beaten egg then place in the oven.
Bake for 25-30 minutes until the pastry is golden brown and crisp.
Remove from the oven, slice and serve.
While the salmon is baking, make the sauce. Melt the butter in a small saucepan then add the garlic and cook for a few seconds. Pour in the cream and add the dill. Bring to a simmer and cook for 5 minutes. Season with salt, pepper and lemon juice and remove from the heat. Serve the sauce with the baked salmon en croute.