Blend remaining juices and onion/garlic in the crockpot with an immersion blender for your gravy until you reach desired consistency.
Serve gravy with roast beef. If I happen to have chives or parsley growing in my garden, I'll usually snip some off to serve on top of the cooked roast, too.
Enjoy!
Recipe by Primally Inspired at https://www.primallyinspired.com/crockpot-balsamic-roast-beef/