Potato, Mushroom & Caramelized Onion Pierogi

INGREDIENTS

• 4 - 6 tablespoons unsalted butter
• 2 yellow onions, chopped
• 3/4 pounds potatoes for mashing
• 1 egg, lightly beaten
• Sour cream or Full-fat or Greek yogurt for serving
• 2 1/4 cups flour + more for kneading
• 2 cups full fat plain or Greek yogurt
• Salt and freshly ground black pepper
• 1 teaspoon salt
• 1 pound white mushrooms, trimmed and finely diced