In the bottom of a 3½-quart or larger slow cooker, place beer or beef broth and tomato paste, stirring well to mix.
Add the onion, mushrooms, carrots, salt, pepper and garlic, stirring well to mix.
Add beef tips; stir to coat with the sauce.
Cover and cook on low for 9 hours.
Remove beef to a serving platter; cover to keep warm.
Turn slow cooker to high.
In a small mixing bowl, combine cornstarch and water until smooth.
Add cornstarch mixture to sauce, stirring well to mix.
Cook an additional 15 minutes until sauce thickens.
Stir in lemon juice, pour sauce over beef and serve.