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 sweet potato cubes, sliced leeks, carrots, and minced garlic.
Pour in the beef broth. Stir in the salt, black pepper, fresh thyme leaves, and bay leaf.
Cover and cook on low for 7 to 8 hours until the beef is tender and the vegetables are soft.
Remove the bay leaf. Stir in the lemon zest and lemon juice right before serving.