In a large pot, heat the olive oil over medium heat. Add the beef stew meat and brown on all sides, about 5-7 minutes.
Add the chopped onion and minced garlic to the pot, cooking until the onion is translucent, about 3-4 minutes.
Stir in the carrots, celery, green beans, and potatoes. Cook for another 5 minutes, stirring occasionally.
Pour in the diced tomatoes and beef broth. Add the dried thyme, dried oregano, salt, and pepper. Bring the mixture to a boil.
Reduce the heat to low, cover, and let simmer for 1 to 1.5 hours, or until the beef is tender.
If using, stir in the chopped kale or spinach during the last 5 minutes of cooking.
Taste and adjust seasoning if necessary before serving.