Peel any bruised outer layer from the cabbage. Cut about 1 inch off the bottom.
Drop the cabbage into the water. wait about 5 minutes, then use tongs to peel off the outer layer and put it on a clean work surface. Continue until you have at least 18 leaves, swapping with the other cabbage if the leaves get too small.
Preheat the oven to 350 F.
In a large bowl, mix together the turkey, potato, onion, pepper, garlic, dill, parsley, egg, and potato flakes. Season with salt and pepper and mix well until combined.
In a mixing bowl, stir together the tomatoes, vinegar, and paprika, and season with salt and pepper.
Spread a thin layer of the tomato sauce into the bottom of the pan.
One by one, add a scoop of the turkey filling into a cabbage leaf and roll it up, starting with the stem side, and ending with the leafy side, tucking in the edges as you go.
Place the roll seam side down in the pan and repeat until you have no more filling. You should tuck the rolls tightly in the pan.
Top the rolls with the remaining sauce.
Bake at 350 for about an hour until the turkey is cooked and the sweet potato is tender.