The origins of tonkotsu broth can be attributed to the rise of ramen culture in post-World War II Japan. During that time, a restaurateur named Nagahama Yasuji opened a ramen shop called "Ajitoya" in Hakata, Fukuoka. Ajitoya specialized in serving a new style of ramen that featured a creamy and flavorful pork-based broth, which eventually became known as tonkotsu ramen. Nagahama Yasuji developed the technique of boiling pork bones for long hours to extract their rich flavors and create a hearty broth. The resulting tonkotsu broth was different from the lighter soy-based broths that were more commonly used in ramen at the time. The Hakata-style tonkotsu ramen quickly gained popularity in Fukuoka and eventually spread to other parts of Japan.
Optional Step
Optionally, you can remove ginger and onion and continue to boil until there is no cartilage or soft tissue left on the bones and they look bare, adding water as needed to keep the water level high, approximately 1 hour more. This will result into more rich broth.
Using soy sauce base tare will ruin the milky white presentation of the broth. However from taste perspective I really prefer shoyu tare.