Place rinsed barley in a medium saucepan over medium heat. Stir while cooking for 1 minute. Add 2 cups of water, bring to a boil, reduce heat to low and cover. Let simmer for 30 minutes.
Slice butternut squash in half lengthwise and scoop out the seeds. Lay face-up on a baking tray and cook for 45 minutes or until fork tender.
Add celery, onion, mushroom and bell pepper to barley. Cover and continue to cook for an additional 10 minutes.
Thoroughly combine all balsamic glaze ingredients in a small bowl.
Remove roasted squash from oven and generously add filling. Return to oven for 15 minutes.
Remove from oven, generously drizzle with glaze and enjoy!