Warm olive oil and butter in a large Dutch oven over medium heat. Once butter is melted, add onion and garlic and saute until translucent and fragrant, about 5 minutes.
Add celery, carrot and butternut squash and saute another 5 minutes.
Add bay leaf, chicken stock and Parmesan rind (if using) and bring to a low boil.
Stir in kale, white wine and sausage and let simmer until kale is wilted and sausage is heated through. Season with salt and pepper to taste and keep warm on low heat until ready to serve.
Once your soup is prepared, make your crispy Brussels sprouts. Heat olive oil and bacon grease (if using) in a large, deep saute pan over high heat. Once oil is sizzling, drop in sliced Brussels sprouts. If the sprouts are going to be crowded in your pan if you use a whole batch, you can split them up into two batches.
Fry Brussels sprouts until they look frazzled with dark brown edges. Strain out of oil and place on a plate lined with a paper towel to drain and crisp up.
When you’re ready to eat, ladle the soup into bowls and top with Brussels sprouts.