In a large skillet, heat olive oil over medium-high heat. Add the beef cubes and brown on all sides, about 5-7 minutes. Transfer the beef to the crockpot.
In the same skillet, add the onions and garlic, sautéing for 2-3 minutes until softened. Add to the crockpot.
Add the carrots, potatoes, celery, green beans, diced tomatoes, beef broth, tomato paste, Worcestershire sauce, thyme, rosemary, salt, and pepper to the crockpot. Stir to combine.
Cover and cook on low for 8 hours or on high for 4 hours, until the beef is tender and the vegetables are cooked through.
If you prefer a thicker stew, mix the cornstarch with water in a small bowl to create a slurry. Stir this into the stew during the last 30 minutes of cooking.
Taste and adjust seasoning if necessary before serving.