1 (17.25-oz) package frozen puff pastry, thawed in fridge
Mix curry powder, cumin powder, turmeric powder, sea salt, sugar and black pepper in a small bowl.
Heat oil in a large skillet over medium heat until warm. Add white onion. Cook and stir until the translucent and the edges are slightly brown, 8 to 15 minutes. Add garlic. Cook another minute until softened.
Add the curry mix. Cook for a minute or so, until fragrant but not burned.
Add chicken stock. Cook and bring to a boil.
Place leftover mashed potato in a large bowl and pour in the curry mixture. Mix with a spatula until the liquid is absorbed. Let cool completely.
To make egg wash, beat the egg with 1 tablespoon water in a small bowl.
Unfold puff pastry. Cut the sheet into 9 squares. Place about 2 to 3 tablespoons of the filling in the center of each square, fold diagonally to form a triangle. Press the edges together and press with a fork to seal. Transfer the pieces onto a baking sheet, two fingers apart between each.
Place the puff pastries into the fridge to let cook for at least 15 minutes.
Preheat oven to 400 °F while cooling the puffs.
When the puffs are ready to cook, gently brush a thin layer of egg wash over each one and then place into the oven. Bake for 15 to 18 minutes, or until golden brown.
Let cool on the baking sheet for 10 minutes and then transfer onto a wire rack.