Heat the oil over medium-high heat in an 5-qt soup pot or Dutch oven. Season the lamb with salt and pepper and add it to the pot in batches. Sear the meat until brown then set it aside.
Add the onions and garlic to the pot. Sauté until the onions begin to soften, about 3-4 minutes, and then add the carrots and celery. Sauté for a few more minutes.
Add the red wine, beef broth, rosemary, and lamb. Bring the mixture to a simmer and cover. Continue to simmer for 90 minutes, or until the lamb becomes fork tender.
Add the butternut squash and simmer until it's cooked through, about 8-10 minutes. Make a slurry with the cornstarch and water and stir it into the stew. Return the stew to a simmer and add the balsamic vinegar.
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