In a large skillet, heat olive oil over medium-high heat. Add the beef cubes and brown on all sides, about 5-7 minutes. Remove the beef and set aside.
In the same skillet, add the onion and garlic. Sauté until softened, about 3-4 minutes.
In a slow cooker, combine the browned beef, sautéed onion and garlic, carrots, potatoes, red and green bell peppers, diced tomatoes, beef broth, Worcestershire sauce, tomato paste, thyme, and rosemary. Season with salt and pepper.
Stir everything together until well combined. Cover and cook on low for 7-8 hours or on high for 4-5 hours, until the beef is tender.
Once cooked, taste and adjust seasoning if necessary. Serve hot, garnished with fresh parsley.