Generally the thin sliced is meant for quick grilling or frying.
Grill or fry over high heat, about like you would for bacon.
Serve with steamed white rice, kimchi, loose leaf lettuce, and ssamjang (a seasoned red pepper paste), or with a sesame oil, salt, and black pepper dipping sauce.
Some other things that can be done:
Smear the slices with doenjang (Korean Miso) before grilling.
Cut into bite sized pieces, fry, and wrap with long whole leaf style kimchi, steamed napa cabbage leaves, or fresh perilla leaves.
Wrap the slices around hot or mild peppers (your choice), and a clove of garlic, then deep fry or pan fry for a couple of minutes.
Bulgogi? Since it's thinly sliced, and from H-Mart, I'm guessing that was its intended use. For pork belly in general, I like to use it in braised dishes, either in Asian cooking or in a nice vat of bigos (Polish stew).
What else did you buy, or are you planning to make with the pork belly?