Mix 3 ½ cups flour and 1 teaspoon salt in a large bowl; make a well in the center. Pour ¼ cup vegetable oil and 1 cup + 2 tablespoons warm water into the well. Stir to form dough and clean off the spoon.

Knead dough on a work surface until smooth and soft. Wrap in plastic and let rest for 1 hour. Refrigerate as needed.

Melt butter in a skillet over medium heat. Sauté diced onion and salt until golden brown. Turn off heat and let cool. Reserve in a bowl.

Boil peeled and quartered russet potatoes in salted water; simmer until tender (approx. 20 minutes). Mash until smooth and cool to room temperature.

Mix farmer’s cheese with salt, pepper, cayenne, and 2 tablespoons buttered onions. Fold in mashed potatoes until combined.

Roll dough thinly (1/8 inch), cut 3 ½-inch circles, and fill with 2 tablespoons filling. Seal edges, ensuring no air is trapped.

Boil pierogi in salted water for 2 to 3 minutes per batch. Drain and transfer to melted butter in a skillet. Pan-fry until golden brown.