*If you do not have buttermilk you can make an equivalent substitute by combining 1 cup milk with 1 tbsp of vinegar. Stir and let rest for 10 minutes before using. For this recipe I used 1 cup skim milk and 1 tbsp of red wine vinegar to mimic the buttermilk.
Preheat oven to 350F. Grease a baking dish or lasagna pan. Pour the tomatoes (with juice) into the pan and scatter the sliced onions over the top. Bake for 50 minutes to 1 hour until the mixture is thick and the onions begin to brown.
Transfer the mixture to a blender. Add the broth, water, paprika and chili powder and puree until smooth. Heat the soup mixture in a pot over medium-low heat until heated through. Add the buttermilk and stir to combine. Ladle into 4 soup bowls, or mugs, and garnish with Greek yogurt and avocado slices or other toppings of choice.
Review (out of 5): *****