Melt the butter in a large skillet over medium-high heat. Add your sliced mushrooms, sprinkle with a pinch of salt and pepper, and cook them for about 3-4 minutes until they're nice and browned. Transfer these flavorful mushrooms to your slow cooker.
Season your beef pieces with salt and pepper. In the same hot skillet, brown half of the beef quickly on all sides to lock in the flavor. Move the browned beef to the slow cooker, making sure to scrape in all those delicious pan juices. Repeat this step with the remaining beef.
Pour in the beef broth, minced garlic, tomato paste, Worcestershire sauce, and onion powder into the slow cooker with the beef and mushrooms. Give it a good stir to combine everything.
Cover your slow cooker and let it do its magic! Cook on the LOW setting for 8 hours (or on HIGH for 5 hours) until the beef is incredibly tender and easily falls apart.
Once cooked, carefully scoop out about 1 cup of the hot juices from the pot into a separate bowl. Whisk in the glucomannan (or xanthan gum) until smooth, then pour this mixture back into the slow cooker. Stir well until the gravy thickens to your liking.
Taste and adjust any seasonings if needed. Serve your healthy and hearty beef tips over creamy mashed cauliflower or cauliflower rice for a complete, low-carb meal.