Can I save an undercooked loaf of pumpernickel? I made this pumpernickel recipe from YouTube (recipe below) and I think I underbaked it: it has a little of that unpleasant chalky taste of raw flour.

Can I rescue this loaf or do I need to start from scratch?

(For science, I plan to put it back in the oven wrapped in foil for a few more hours and I'll update with the results, but I'd like to see if anyone has other suggestions or experience.)

300 g rye flour
150 g wholemeal flour
 80 g bulgur wheat
  1 tsp salt

500 mL warm water
  1 tbsp molasses
  1 tbsp vegetable oil

1. Mix dry ingredients
2. Mix wet ingredients
3. Combine and stir until totally incorporated
4. Fill greased and lined loaf tin with dough
5. Cover with plastic wrap and let sit overnight
6. Remove plastic wrap, cover tightly with aluminum foil
7. Bake 6 hours at 230°F over tray of water
8. Remove foil and bake 15 more minutes
When a loaf is baked the interior steams. The water in the dough reaches boiling point and gelatinizes the starches in the flour. After baking the dough has largely dried out. While this might be less true for very moist breads such as pumpernickel, I doubt that baking again will have much effect on the remaining uncooked flour. So I'm afraid there is not much you can do at this point.

Toasting may make the bread more palatable, but it won't make the essential chemical change that you need.