Slice onion into ¼” slices.  Separate slices.  Add butter to a medium skillet over medium low heat.  Add onions and sugar. Cook onions for 15-20 minutes, stirring around occasionally, until tender and caramelized.  Remove from heat.

Cook bacon to chewy-crisp, reserve 2 tsp fat.  Set bacon aside.

Combine ground pork, Worcestershire sauce, half (1.5tsp) of reserved bacon grease and seasonings.  Fluff with a fork, don’t overmix.  Shape into 4 patties.  (I shape mine to suit the shape of the slices of bread.)  Preheat a large skillet over medium high heat.  Add oil and bacon grease to skillet, wait until it shimmers—you’ll know it is hot enough.  Fry patties until they are cooked to medium well doneness (roughly 160 degrees is great if you have an instant read thermometer).

Clean out skillet, return to heat, reducing to medium or medium low for toasting the sandwich (some stovetops are hotter than others).  Spread outside of all 8 bread slices with butter, spread mustard on inside of 4 bread slices.

Set butter and mustard slices butter-side-down in warmed skillet. Lay a slice of cheese, then the pork patty, 2 slices of bacon, onions and additional slice or handful of grated cheese.  Top with more cheese and the other slice of bread.   Flip when golden and cook other side until cheese is melted and bread is golden brown and toasted.

Serve immediately.  Makes 4 generous sandwiches.