Classic Potato Cheese Perogi: Grandma's Secret Recipe

Ingredients

• 2 cups (250g) all-purpose flour, plus more for dusting
• 1 teaspoon (5g) salt
• 1 large egg
• ¾ cup (175ml) warm water
• 2 tablespoons (30ml) vegetable oil
• 1 1/2 lbs (680g) russet potatoes, peeled and quartered
• 4 tablespoons (56g) unsalted butter, divided
• ½ cup (120ml) milk, warmed
• ½ cup (115g) shredded cheddar cheese (or farmer's cheese/twaróg)
• Salt and black pepper to taste
• 2 large yellow onions, thinly sliced
• 2 tablespoons (30ml) olive oil
• 1 tablespoon (15g) unsalted butter
• 1 teaspoon (5g) brown sugar
• Pinch of salt