Whole Stuffed Cabbage

Name: Whole Stuffed Cabbage
Ingredients: 1 large egg, plus 1 egg white; ⅓ cup kasha (roasted buck- wheat kernels), such as Bob's Red Mill; ⅔ cup boiling water; Kosher salt and freshly ground pepper; 1 small Savoy cabbage; 5 tablespoons extra-virgin olive oil; 1 onion, finely chopped (2 cups); 3 medium carrots, shredded (2 cups); ½ cup finely chopped fresh parsley, plus more for serving; ½ cup finely chopped fresh dill, plus more for serving; 1 pound ground turkey, preferably dark meat; 4 teaspoons unbleached all-purpose flour; 1 can (28 ounces) diced tomatoes; Sour cream, for serving