Brown the ground beef in a skillet over medium heat, breaking it up as it cooks. Drain any excess fat.
Add the browned beef to the slow cooker along with the cubed butternut squash, diced onion, and minced garlic.
Stir in the dried sage, beef broth, diced tomatoes, salt, and black pepper.
Cover and cook on low for 6 to 7 hours until the squash is tender.
Stir the stew before serving to combine the ingredients evenly.