Place the beef tips and flour in a large zip-top bag. Close the bag and shake to coat the meat.
In a heavy skillet, melt butter. Brown the meat in batches on all sides.
As the meat is seared, transfer to the crockpot.
Add 1 cup of the beef broth to the skillet and scrape the bottom with a wooden spoon to deglaze the pan.
Pour the broth over the meat in the slow cooker.
Add the remaining 1 cup beef broth, onion, mushrooms, pepperoncinis, Worcestershire, garlic powder, dry mustard, paprika, thyme, salt, pepper, and bay leaf to the crockpot.
Cover and cook on low for 6 hours.
Whisk together the cornstarch and cold water. Mix into the slow cooker and cook an additional 30 minutes.
Serve beef tips with gravy.