In a large pot, add diced onion, butternut squash, vegetable stock, chili powder, sugar, cumin, black pepper, salt, coriander, nutmeg, and bay leaf. Bring to a boil and reduce heat to medium low. Simmer, covered, for 45 minutes.
After cooking, carefully remove the bay leaf from the pot. With an immersion blender*, blend all of the ingredients in the pot until smooth. Stir sour cream into the soup. Season to taste with additional salt as needed.
Set soup aside until the croutons have been prepared.
To Make the Pan-Fried Sage Croutons
Heat olive oil in a cast iron skillet over medium heat. When the oil is hot, add the bread cubes, chopped sage, salt, and pepper. Cook, stirring occasionally, for approximately 10 minutes until the bread starts to become toasted on the outside. Serve the croutons on top of the soup. Enjoy!
A countertop blender can be used in place of an immersion blender. However, make sure not to fill the blender more than half full before blending. The hot contents of the soup will expand slightly while blending.