For the pickled onions: Add water and vinegar to a medium sized pan and add sugar, mustard seeds and garlic. Bring to boil and let the sugar dissolve.
Meanwhile slice onions into bite sized pieces and add to a jar.
Pour vinegar mixture overtop until you fill up the jar. Let cool and then add to the fridge. Let sit overnight.
When you're ready to eat: shred cheese and add to bread along with bacon jam.
Add pickled onions and apples slices to the top and distribute evenly.
Coat pan in butter and add sandwich on medium heat. Add a little more butter before flipping, about 4 min per side or until cheese gets melty and bread starts to crisp. Enjoy!