Prep. Preheat the oven to 200ºF. Cover a sheet pan with foil and lightly grease it with cooking spray.
Coat bananas with lemon juice. Toss the banana slices in lemon juice. Arrange the bananas in a single layer on the prepared baking sheet.
Bake. Bake for 1 hour; check to see if the tops are firm and starting to turn color. If so, flip the slices with a metal spatula and continue to bake for 1 hour longer or until desired crispness (see note).
Cool. Remove from the oven and transfer the banana chips to a wire rack to cool completely. The longer they cool, the crunchier they will get. Store in an airtight container.
Notes
Try to cut all the banana slices evenly.
Check on the banana slices from time to time as oven temperatures do vary. The thickness of the slices also matters. Pull any done banana chips from the oven as needed.
If you find the bananas are sticking and aren't able to flip easily, keep baking for another 20 to 30 minutes before flipping.
If you remove the banana chips from the oven too soon, they will not start to crisp; if this does happen, return the chips to the oven for an additional 15 minutes.