Cheerio,

While there are those settings in the slicer that is not the normal method to fix this. 

You need to recalibrate the printer.  So that the printers internal 0,0 match the real location on 0,0.

I'm sorry, but recalibration doesn't fix it. I did a recalibration, and it made no difference. Recalibration won't help because the printer cannot detect the actual edges of the print sheet on the print bed. The print bed itself doesn't move left/right, so its position is not adjustable. The distance from the left-most position of the extruder to the left edge of the print sheet is something that needs to be manually set. I suspect the problem is that an MK3.5 that is upgraded from an MK3 is slightly different from a native MK3, but the firmware/slicer (not sure which one is the real culprit) is hand-coded to use a native MK3. That's why it's off by 1mm.

You need to recalibrate the printer.  So that the printers internal 0,0 match the real location on 0,0.  The most common cause of an offset X is that something is stopping the detection in the wrong place when it is homing, often the cable bundle.  All it takes is a misplaced cable tie on the bundle when you first calibrate the printer and its off my a couple of mm.  

It seems that my X-axis in my printer is misaligned with what PrusaSlicer thinks the axis is.  If I have a print that's almost as wide as the print bed, I need to nudge it 1-2mm to the left in order for it to print without the print head bumping into the right side of the printer (and thereby misaligning the layers).

I don't think I can change the printer itself (I can't move the print bed left or right) so I need to tell PrusaSlicer about the offset.  How do I do that?