Put the chicken broth and tomatoes in a large Dutch oven or pot, cover and bring to a boil over medium-high heat. Meanwhile, put the onion, olives, mint, oregano and pumpkin pie spice in a food processor; pulse until very finely chopped. Add half of the onion mixture to the pot. Add the bread to the food processor with the remaining onion mixture and pulse to form wet crumbs. Transfer to a large bowl, add the beef and mix with your hands until just combined. Form into 20 small meatballs and add to the pot. Cook, turning the meatballs once, until just firm, about 3 minutes. Gently stir the butter beans and zucchini into the stew; cook until the zucchini is tender, about 5 minutes. Add lemon juice to taste.