Preheat oven to 375°F.
Grease six (12-ounce) ramekins or bowls, place on a rimmed baking sheet.
Heat butter in a large skillet over medium heat.
Add onion and cook, stirring occasionally, until tender, 5 to 7 minutes.
Add poultry seasoning and stir.
Sprinkle flour over onion and cook, stirring, 1 minute.
Slowly stir in milk.
Bring to a simmer; cook, stirring constantly, until thick and bubbly, 2 to 3 minutes.
Stir in turkey and your choice of leftover veggies. Season with salt and pepper.
Ladle into prepared ramekins, let cool. This is important or your pastry won't puff.
Roll out thawed puff pastry on a floured surface to a ¼ -inch thickness.
Cut out six circles that are 1-inch larger than ramekins or bowls being used.
Top each ramekin with a puff pastry circle, lightly crimping edges over sides to seal.
Beat the egg with a teaspoon of water or milk. Brush the pastry with egg.
Bake until puffed and golden brown, 20 to 25 minutes.