Heat the olive oil in a skillet over medium-high heat. Add the beef cubes and brown them on all sides for about 5 minutes.
Transfer the browned beef to the slow cooker. Add the onion, carrots, garlic, tomato paste, salt, pepper, and bay leaves.
Pour in the beef broth and stir to combine everything. Cover and cook on low for 6 hours.
Add the butternut squash cubes and chopped sage. Stir gently, then cover and cook for 2 more hours until the squash is tender and the beef is soft.
Remove the bay leaves before serving.