In a large pot or Dutch oven, heat vegetable oil over medium heat. Add the chopped onion and garlic. Stir often until the mixture softens and starts to caramelize, about 5 minutes. This step builds a deep, flavorful base.

Add sweet Italian sausage to the pot, breaking it apart with a wooden spoon. Season with salt and pepper. Cook until browned and fragrant, letting the rendered fat mingle with the onions and garlic.

Sprinkle in ground sage and dried thyme. Stir for about 1 minute to toast the herbs and release their aroma. Add sliced butternut squash and bay leaves.

Pour in the chicken stock and balsamic vinegar. Bring to a boil, then reduce the heat to low. Let it simmer for 30 minutes, stirring occasionally, until the squash is tender.

Remove the bay leaves. Blend the soup using an immersion blender until it reaches your desired texture—smooth or slightly chunky. Add in fresh or frozen spinach and just cook until wilted. Taste and adjust seasoning.

Ladle the soup into bowls. Drizzle with a touch of cream or olive oil and serve with crusty bread.