In a large skillet, heat the olive oil over medium heat.

Add the chopped onion and cook until soft and translucent, about 5 minutes.

Incorporate minced garlic and cook for an additional minute until fragrant.

Add the ground Italian sausage. Using a wooden spoon, break it into smaller pieces and cook until thoroughly browned, around 5 to 7 minutes. Drain excess fat if needed.

Transfer the sausage mixture into a slow cooker. Add the diced butternut squash, chopped carrots, and celery.

Pour in the broth and sprinkle in the thyme, basil, sea salt, and black pepper. Mix everything well to ensure even distribution.

Set the slow cooker to low and let it simmer for 6 to 8 hours, or on high for 3 to 4 hours, until the vegetables are tender.

If desired, stir in the chopped spinach and balsamic vinegar during the last 30 minutes of cooking.

Before serving, taste the soup and adjust seasoning as necessary to suit your palate.