Combine coffee concentrate, two cups of beef broth, salt, and pepper.
Add beef roast to slow cooker. Add garlic and onions on top. Pour broth mixture over top. Secure lid and cook on low for 8 hours.
Remove beef from slow cooker and let rest while you make the gravy.
Skim off fat. Ladle liquid into a sauce pan over medium heat. Once it starts to bubble, carefully mix in cornstarch while whisking. Allow to thicken then whisk in remaining beef broth.
Slice roast into ½inch pieces and serve with a ladle of gravy.