In a medium pan over medium-high heat, add the slices of SPAM and pineapple. If you're making multiple sandwiches, sear the pineapple first and move it to a plate to rest, but if you're just making one sandwich, everything should cook at the same time for the most part.
Cook the slices of meat and fruit for about 4 minutes on the first side and then flip and cook for an additional 3 to 4 minutes or until both sides of the SPAM have a nice darkened sear. Remove the meat and pineapple to a plate until you're ready to make the sandwich.
Once the meat and fruit have been removed from the pan, you can slice and toast your bun lightly in the pan for about 1 minute per side. Remove the bun from the pan and turn off the heat.
Add a spread of mayo to the bottom bun and then top it with slices of SPAM and pieces of seared pineapple.
Drizzle the meat and pineapple with some Japanese BBQ sauce, and then top that with a handful of crispy onions.
Add mayonnaise to the top bun if desired and close the sandwich. Serve and enjoy.