Marinate the pork with 1/4 cup (60 milliliters) of kecap manis and the white pepper in a large mixing bowl for about 1 hour.
Heat the oil in a large pot over medium-high heat until shimmering hot.
Add the shallots, garlic, and ginger and fry until fragrant and wilted, about 1 minute.
Add the pork and stir and cook until no longer pink, about 3 minutes.
Add the salt, sugar, water, and remaining 1/4 cup (60 milliliters) of kecap manis. The liquid should reach about three-fourths of the way up the pork. Add more water if necessary. Bring to a boil.
Reduce the heat to medium and simmer, covered, for 30 minutes.
Add the potatoes and cook until they can be easily pierced with a fork, 10-15 minutes. If you would like the pork to be even more tender and you have time, simmer for another 15-30 minutes before adding the potatoes. Taste and adjust seasonings if desired.
When the pork and potatoes are ready, add the onion and remove from heat.
Shower with fried shallots and serve with steamed rice and a vegetable side dish.