Put tonkotsu, sake, mirin and pork belly into a pot, simmer until the alcohol has cooked off.
Then add everything else except the white pepper and shiitake. Keep it simmering until its almost reduced by half.
Finally add the white pepper and shiitake and simmer until you are left with about 500 ml of liquid.
Take it off the heat and let the everything sit for about 30 mins.
Strain with a fine mesh sieve. I put it in a container and freeze it and take it out when needed.