Place all sauce ingredients in a small saucepan and bring to a boil.
Lower to medium temperature and simmer for 15 minutes.
With an immersion blender, puree mixture for 1 minute.
Pour through a mesh strainer and discard solids.
Store refrigerated in an airtight container.
Season frozen burger evenly with salt and pepper and place burger on a 350-degree grill or flat top.
Once the burger begins to form juice pockets on the top (approximately 3.5 minutes), the burger is ready to flip.
* Do not press on the burger as it will release important juices and lead to a dry finished product.
* In the final stages of cooking, while burger is still on the grill, baste burger on both sides with bbq sauce until sticky.
Remove burger from grill and place on bun bottom, top with onion, and Napa cabbage.
Spread top bun with kewpie mayonnaise, place on burger and secure with pick.