In a large pot, heat olive oil over medium-high heat. Add the beef stew meat and cook until browned on all sides. Remove the beef and set it aside.
In the same pot, toss in the chopped onion, garlic, carrots, celery, and bell pepper. Sauté the veggies for about 5 minutes until the onions are soft and translucent.
Add the diced potatoes, green beans, diced tomatoes, and browned beef back into the pot. Stir to combine.
Pour in the beef broth, followed by the dried thyme, salt, and pepper. Stir well.
Lower the heat, cover the pot, and let everything simmer for about 1.5 hours, allowing the flavors to meld.
Once the soup has simmered, add in the chopped kale or spinach, and continue to cook for another 15-20 minutes, until the greens are tender.
Serve and enjoy your delicious, hearty vegetable beef soup!