Heat avocado oil in a large skillet over medium-high heat.
Add beef sirloin tips in batches and sear on all sides until browned, about 2-3 minutes per side. Transfer browned beef tips to the crock pot.
In the same skillet, add diced onion and cook for 3-4 minutes until softened.
Add minced garlic and cook for 1 minute until fragrant.
Pour beef broth into the skillet and scrape up any browned bits from the bottom with a wooden spoon.
Stir in Worcestershire sauce, dried thyme, dried rosemary, salt, and black pepper.
Pour the mixture over the beef tips in the crock pot.
Cover and cook on low for 6 hours until beef is tender.
About 30 minutes before serving, melt butter in a small bowl and whisk in xanthan gum until smooth.
Stir the butter and xanthan gum mixture into the crock pot to thicken the gravy.
Add heavy cream and stir to combine.
Cook uncovered on low for an additional 30 minutes until gravy is thickened.
Serve hot over cauliflower rice or mashed cauliflower.