Jewish Potato Knish

Name: Jewish Potato Knish
Ingredients: For the Knish Dough:; 8 ounces (2 sticks) unsalted butter, softened; 8 ounces (1 brick) cream cheese, softened; 1/2 cup sour cream; 3 1/2 cups all-purpose flour; 1 teaspoon kosher salt; For the Potato Filling:; 3 large potatoes (about 1 3/4 pounds), peeled, cut into 1-inch chunks, boiled in well-salted water, cooled and passed through a ricer); 1 medium onion (about 1 pound), chopped and sautéed; 1 large egg, beaten; 1/8 cup pre-made instant mashed potatoes (prepared according to package directions); 1 teaspoon kosher salt, more to taste; Optional: 1/2 teaspoon ascorbic acid (or vitamin C powder, to keep the potatoes from turning dark); 1/2 teaspoon garlic powder; 1/2 tablespoon hot sauce (Tabasco); Optional: 4 ounces (1 cup) cheddar cheese, finely shredded; For the Egg Wash:; 1 tablespoon water; 1 drop of yellow food coloring, optional