In a large pan over medium-high heat, saute the pork belly until just cooked. This should take about 3 minutes. Remove the pork to a plate.
In the same pan (no need to add oil since there should be some fat/oil in the pan from the pork belly), add the sliced onion and saute until it starts to brown, about 4 minutes.
Then add the green onions and cook for 1 more minute. Add the kimchi and cook for 2 more minutes.
Add the cooked pork belly back into the pan and saute for 1 more minute.
Add the soy sauce and sesame oil. Stir to mix evenly. Turn off the heat. Plate and eat with rice!