Heat a nonstick skillet over medium heat until hot. Add the bacon to the skillet and cook until not quite crisp, about 2 minutes per side; remove to a plate. Add the onions to the skillet, season with a pinch of salt and pepper and cook until golden, about 10 minutes. Transfer to a paper towel-lined plate to drain any fat. Wipe the skillet and place it back on the heat.
Begin building the rest of the sandwich: Spread peanut butter on both slices of bread, then spread jam on both slices as well. Place the manchego on one of the slices, then cut the provolone in half and place on top of the manchego. Cut the Swiss cheese in half and place on top of the jam on the other slice. Pile the onions on top of the provolone cheese, followed by the pieces of bacon. Make the sandwich by closing the 2 filled sides of bread together and press slightly to help it all stay together.
Spread the top of the sandwich with half the mayonnaise. Place the sandwich into the skillet mayonnaise-side down. Spread the rest of the mayonnaise on the top of the sandwich. Fry until the outsides of the sandwich are golden brown and the cheese in the middle is melted, 3 to 4 minutes each side. Cut in half and serve.