Heat a dutch oven over medium-high heat. Add olive oil, and when hot, add onions and saute for about 4 minutes. Be sure to season with a little salt and pepper during this time. Add garlic and continue to saute for 2 more minutes. Add the tomato paste and saute it in the olive oil for about 2 minutes to help it soften. Add green beans, broth, diced tomatoes, oregano, and plenty of salt and pepper. Bring to a boil, then reduce heat to medium-low and let simmer for 3-4 hours, or until the green beans are nice and tender. When green beans are very tender, remove from heat and uncover. Add the juice of 1/2 lemon, stir. Taste and add salt and pepper, to taste, if needed. Serve and enjoy!