Preheat oven to 425 degrees F.
Heat a large pot over medium heat and add olive oil and butter. Add onions with pepper flakes and a pinch of salt and stir to coat, then cook until softened, about 5 minutes. Add in garlic and cook for another minute. Increase heat slightly and add bourbon, stirring well. Let cook for 2-3 minutes.
Add sweet potatoes, stock and water to the pot. Cover and let simmer for 25 minutes, until sweet potatoes are soft.
While sweet potatoes are boiling, add bread cubes to a baking sheet. Toss with cinnamon and sugar, then brown butter, and bake for 8-10 minutes until golden. Remove and set aside until soup is finished.
After sweet potatoes are soft, carefully pour the entire mixture into a high-powdered blender and pureed until smooth and creamy, about 5 minutes. Additionally, you can use an immersion blender but the soup may not be as smooth. Transfer creamy soup back into the pot over low heat. Stir in half and half, salt and pepper, then taste and season additionally if needed. Serve with croutons on top!