Slice onions thinly and caramelize in a skillet over medium heat with a splash of water or vegetable broth for 10-12 minutes.

Cook potatoes in boiling water until tender, around 15-20 minutes, then mash and combine with caramelized onions and vegan cheese.

Flour the work surface, roll out puff pastry to ¼ inch thickness, and cut into squares about 4x4 inches.

Place a spoonful of filling in the center of each square, fold over to form a triangle, and seal the edges with a fork.

Preheat oven to 400°F (200°C) and arrange filled pastries on a baking sheet lined with parchment paper.

Bake for about 20 minutes until golden brown and puffed.

Allow pastries to cool on a wire rack for a few minutes before serving warm.