Potato pierogies

For the Pierogi dough: ▢ 4 cups all-purpose flour ▢ 1 teaspoon kosher salt ▢ 1 cup whole milk ▢ 2 eggs, lightly beaten ▢ 1/4 cup sour cream For the Pierogi Filling ▢ 5 large russet potatoes, peeled and quartered ▢ 1 teaspoon kosher salt ▢ 3 tablespoons butter ▢ 3 large vidalia onions, diced ▢ 1 1/2 cups large curd cottage cheese ▢ 8 ounce cream cheese, room temperature