In a large bowl, combine flour and salt. Make a well in the center and add the egg, water, and vegetable oil.
Mix until a dough forms. Knead on a floured surface for about 5 minutes, or until smooth and elastic. Cover and let it rest for 30 minutes.
In a skillet, melt butter over medium heat. Add chopped onion and cook until soft and golden.
In a bowl, combine mashed potatoes, cheese, and cooked onion. Season with salt and pepper to taste. Mix well.
Roll out the dough on a floured surface to about 1/8 inch thickness. Cut into squares (about 3 inches on each side).
Place a small spoonful of filling in the center of each square. Fold the dough over to form a triangle or rectangle, pinching the edges to seal.
Heat butter in a large skillet over medium heat. Fry the colțunași in batches until golden brown and crispy on both sides, about 3-4 minutes per side.
Enjoy your meal!