Finely dice the pork belly into small cubes.
In a large pot or Dutch oven, heat the vegetable oil over medium-high heat.
Add the diced pork belly and cook until the fat is rendered and the pork is browned and crispy, about 10 minutes.
Add the finely chopped onion to the pot and sauté until softened and golden brown, about 5 minutes.
Add the minced garlic and chopped shiitake mushrooms, and sauté for another 2 minutes until fragrant.
Stir in the soy sauce, dark soy sauce, rice wine, sugar, and five-spice powder. Mix well to coat the pork and aromatics evenly.
Pour in the water or mushroom soaking liquid and bring the mixture to a boil.
Reduce the heat to low, cover, and simmer for about 45 minutes to 1 hour, or until the pork is tender and the flavors are well melded.
If using, add the peeled hard-boiled eggs to the pot during the last 20 minutes of simmering. The eggs will absorb the flavors of the braising liquid.
Enjoy your meal!