Heat the vegetable oil in a large pot over medium heat. Add the sliced pork belly and cook until browned and caramelized, about 5-7 minutes. Remove the pork from the pot and set aside.
Pour in the chicken broth, soy sauce, mirin, and brown sugar. Stir well to combine and bring the mixture to a simmer. Let it simmer for about 10 minutes to allow the flavors to meld together.
While the broth is simmering, cook the ramen noodles according to the package instructions. Drain and set aside.
Add the cooked pork belly back into the pot with the broth. Stir in the baby spinach and let it wilt for a minute.
To serve, divide the cooked ramen noodles between bowls. Ladle the caramelized pork and vegetable broth over the noodles. Top with halved soft-boiled eggs, sliced green onions, nori sheets, and a sprinkle of sesame seeds.