Remove any loose papery skin from the outside of the head of garlic. Cut a slice from the top of the garlic to expose the cloves. Place the garlic on a square of aluminum foil (lined with parchment if desired, but optional). Drizzle with the olive oil and sprinkle with a pinch of salt and pepper. Wrap the garlic in the foil and place in the oven to bake for about 1 hour, 15 minutes.
Remove from the oven and allow the garlic to cool before handling. Remove the cloves from the skin of the garlic by gently pressing the sides of the head of garlic. Be sure to extract all the garlic cloves.
To Make the Soup:
Add the oil and butter to a large Dutch oven or soup pot over medium-low heat. When the butter is melted add the onion and cook, stirring occasionally, unto soft but not browned, about 7-8 minutes.
Add the flour and stir to coat the onion. Cook for an additional 2-3 minutes, stirring frequently.
Add the tomatoes with their juice, chicken broth, garlic, salt, and pepper. Roughly tear the basil leaves and add them to the pot. Increase the heat to medium-high and bring the soup to a simmer while stirring to make sure the flour is not sticking.
Reduce the heat to low, cover and cook for 40 minutes.
Puree the soup with a hand-held stick blender or in small batches in a regular blender. Taste for seasoning and adjust if needed.
Serve warm with a dollop of sour cream if desired.
Note: you can roast several heads of garlic together and store any leftovers in the refrigerator for up to one week.