"The most important part of the layering is making sure the dressing and the spinach (or whatever 'leaf' you choose) don't touch. As long as they stay separate, these salads can be made up to 4 days in advance and will stay fresh in the fridge with a lid on. I make a few jars on Sunday night and just grab one to bring to work everyday. When you're ready to eat, just shake it up."
