In a large skillet, heat the butter over medium-high heat.
Add beef tips in batches and sear until browned on all sides, about 3–4 minutes per batch. You are not looking to cook through just brown.
Place the meat in the slow cooker.
Stir in the sliced onion and minced garlic over the beef.
In a medium bowl, whisk together brown gravy mix, onion soup mix, Coca Cola, beef broth, Worcestershire sauce, onion powder, and garlic powder until smooth.
Pour the mixture evenly over the beef and onions in the slow cooker. Stir gently to coat.
Cover and cook on LOW for 7–8 hours or HIGH for 4 hours, until beef is fork-tender.
Thicken if needed: If you want a thicker gravy, whisk 1 tablespoon cornstarch with 2 tablespoons water, stir into the slow cooker, and cook on HIGH for an additional 15 minutes.
Garnish with parsley if desired. Serve beef tips and gravy over mashed potatoes, rice, or egg noodles.