Melt the butter and cook the sliced leeks, carrots, thyme and sage for 5 minutes. Add the pressed garlic and stir briefly, then pour in the Marsala wine and let the alcohol reduce for a few minutes. Mix in the flour and stir until the vegetables are coated, then stir in turkey or chicken stock gradually, continue stirring until the mixture thickens for about 5-7 minutes, then season with salt and pepper to taste.
Add cooked turkey pieces, frozen peas, mustard, heavy cream and Worcestershire sauce. Bring to a boil and simmer over medium heat for about 10 minutes until the liquid reduces slightly and thickens. Take off the heat, cool for 30 minutes. Taste and adjust the seasoning if necessary. Transfer your pie filling into a 9X13 inch casserole dish. Meanwhile preheat the oven to 200C/400F.
On a floured surface roll out the puff pastry to ¼ inch thick, then cut the pastry into rectangles or leave whole if preferred. Top with cut puff pastry rectangles overlapping them a bit, brush with an egg wash and bake in the preheated to 200C/400F oven for 30-35 minutes until the pastry is golden and the mixture underneath is bubbling. (If you choose not to cut the puff pastry into rectangles, make sure to cut slits into the pastry for ventilation.)