1. Add barley to water or stock. Bring to a boil, lower to a simmer, and cover. Add lemon zest and simmer for approximately 45-50 minutes, or until barley is tender to your taste. Allow to rest for 10 minutes then transfer to a bowl, fluff with a fork and cool slightly. 
2. Add celery, corn, carrot, and peas. Season with salt and cool completely before layering in jars.
3. Layer the salad, the tomatoes, the spinach and the mushrooms in any order that suits your fancy. The photo shows tomatoes followed by mushrooms, barley/corn salad then spinach – a good choice if the salad will be in the jar for long.

If you’ll be serving the salad immediately you may want to make the layers fancier: Place a layer of barley salad on bottom of jar. Arrange half the tomatoes over salad. Arrange half the spinach, then half the mushrooms. Repeat these four layers again, starting with another layer of barley salad, so you have two layers of everything.