Potato and Mushroom Vareniki

Name: Potato and Mushroom Vareniki
Ingredients: 2 1/2 cups all-purpose flour; Kosher salt; 1 large egg; 1/4 cup whole milk; 1/4 cup plus 1 tablespoon seltzer; 2 tablespoons sour cream; 2 large russet potatoes, peeled and cut into 2-inch cubes; Kosher salt and freshly ground black pepper; 1 tablespoon sunflower oil; 10 ounces cremini mushrooms, stems removed and discarded or saved for another use, caps finely chopped; 2 garlic cloves, minced; 2 tablespoons chopped fresh chives; 1 tablespoon fresh thyme leaves; 7 tablespoons unsalted butter; 1/2 pound salo or slab bacon, cut into small cubes; 2 medium sweet onions, such as Vidalia, sliced thinly into half-moons on a mandoline; Chopped fresh dill, for serving; Sour cream, for serving