Combine the curry powder, cumin powder, turmeric powder, sea salt, sugar and black pepper in a small bowl. Mix well and set aside.
Heat oil in a large skillet over medium heat until warm. Add white onion. Cook and stir until translucent and the edges are lightly brown, 8 to 15 minutes. Add garlic. Cook another minute until softened. Turn to medium low heat if the skillet gets too hot.
Add the curry mix. Cook for a minute or so, until fragrant but not burned.
Add chicken stock. It should be brought to a boil immediately. Remove the pan from the stove.
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, then fold diagonally to form a triangle. Press the edges together with your fingers to seal. Transfer the puffs onto baking sheet, two fingers apart between.
Place the puffs into the fridge to let cool for at least 15 minutes while preheating the oven.
Preheat oven to 400 F.
When the puffs are chilled, gently brush with a thin layer of egg wash. Transfer into the oven immediately. Bake for 15 to 18 minutes, or until golden brown.
Transfer onto a wire rack and let cool for 10 minutes or longer.
Serve warm or at room temperature.
Cool the puff pastry completely before storing. Store leftover puffs in a airtight container in the fridge for 2 to 3 days or in the freezer for up to 2 months. Reheat in oven at 150 degrees C (300 F) to let crisp up. You can heat the puffs on a skillet too.