Heat the olive oil in a soup pot over medium heat. Add the onion and celery and cook for 2 to 3 minutes or until the pieces have softened. Add the garlic and cook briefly. Add the tomatoes, serrano pepper, parsley and mint. Turn the heat to low, cover the pan and cook for 15 minutes. Add about 1/2 cup of the stock to the vegetables and puree the vegetables in a blender or food processor (or use a hand blender). Return the ingredients to the saucepan. Stir in the remaining stock and bring the soup to a simmer. Stir in the harissa and season with some salt and pepper. Cook for 5–6 minutes. Whisk in the optional yogurt or milk, and serve the soup hot or let it cool.

Ronnie Fein is a cookbook author, food writer and cooking teacher in Stamford. She is the author of “The Modern Kosher Kitchen” and “Hip Kosher.” Visit her food blog, Kitchen Vignettes, at www.ronniefein.com, friend on Facebook at RonnieVailFein, Twitter at @RonnieVFein, Instagram at @RonnieVFein.