Melt the butter in a large skillet over medium-high heat.
Working in batches, add the beef and sear for 3-4 minutes, turning occasionally until browned on all sides. There's no need to cook it through at this stage—you just want to build a little extra flavor.
Transfer the browned beef to your slow cooker, then scatter the sliced onion and minced garlic over the top.
In a medium bowl, whisk together the brown gravy mix, onion soup mix, Coca-Cola, beef broth, Worcestershire sauce, onion powder, and garlic powder until smooth.
Pour the sauce over the beef and onions, then give everything a gentle stir to coat.
Cover and cook on LOW for 7-8 hours or HIGH for 4 hours, until the beef is fork-tender.
If you'd like a thicker gravy, whisk together the cornstarch and water to make a slurry. Stir it into the slow cooker, switch to HIGH, and cook for another 15 minutes, or until the gravy has thickened.
Sprinkle with fresh parsley, if using, and serve the beef tips and gravy over mashed potatoes, rice, or buttered egg noodles.