Heat the oil in a large pot or dutch oven over medium heat. Add the ham and onion and cook for 5 to 7 minutes, or until the onions are translucent. Add the garlic, cook about a minute more. Add the chicken broth and bring to a boil. Stir in the frozen greens. Bring back up to a boil, reduce the heat, cover, and simmer for 20 minutes.