Place the beef stew meat in a skillet over medium-high heat and cook until browned on all sides, about 5 minutes. Transfer the beef to the slow cooker.
Add the diced onion, minced garlic, diced tomatoes with their juice, beef broth, Italian seasoning, salt, and black pepper to the slow cooker. Stir to combine.
Cover and cook on low for 6 hours.
Add the sliced zucchini to the slow cooker and stir gently. Continue cooking on low for 2 more hours until the zucchini is tender.
Turn off the heat and stir in the torn basil leaves just before serving.