In a large pot, boil the potatoes until soft. Drain and set aside.
In a skillet, heat the vegetable oil over medium heat. Add the chopped onion and minced garlic and cook until softened and lightly browned.
Mash the boiled potatoes in a large bowl until smooth. Add the cooked onion and garlic, and season with salt and pepper to taste. Mix well.
In a separate bowl, combine the flour, baking powder, and salt. Gradually add the vegetable oil and warm water, stirring until a smooth dough forms.
On a floured surface, roll out the dough to about 1/8 inch thick. Cut out 4-inch circles using a biscuit cutter or a glass.
Place a spoonful of the potato mixture onto each dough circle. Fold the dough over and press the edges to seal, creating a half-moon shape. Repeat with the remaining dough and filling.
Preheat the oven to 375°F (190°C). Place the knishes on a greased baking sheet and brush them lightly with vegetable oil.
Bake for 40-45 minutes, or until the knishes are golden brown.
Serve hot and enjoy!