In a large pot, heat olive oil over medium-high heat. Add the beef stew meat and brown on all sides, about 5-7 minutes.
Add the diced onion and minced garlic to the pot, cooking until the onion is translucent, about 3-4 minutes.
Stir in the carrots, celery, potato, and green beans. 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 it simmer for about 1.5 to 2 hours, or until the beef is tender.
If using, stir in the fresh spinach during the last 5 minutes of cooking.
Taste and adjust seasoning if necessary before serving.