Heat the vegetable oil in a large skillet over medium-high heat.
Add the beef tips in batches and brown on all sides, about 4-5 minutes per batch. Transfer browned beef to a slow cooker or large pot.
In the same skillet, add the diced onion and cook until softened, about 3-4 minutes.
Add the minced garlic and sliced mushrooms to the skillet and cook for an additional 3-4 minutes until mushrooms are tender.
Transfer the onion, garlic, and mushroom mixture to the slow cooker or pot with the beef tips.
Pour the beef broth, water, and Worcestershire sauce over the beef and vegetables.
Add the dried thyme, salt, and black pepper. Stir gently to combine.
Cover and cook on low heat for 1 hour 30 minutes, or until the beef is tender.
In a small bowl, whisk together the all-purpose flour and cold water until smooth to make a slurry.
Stir the slurry into the beef and gravy mixture and cook uncovered for an additional 10 minutes, stirring occasionally, until the gravy thickens.
Allow the beef tips and gravy to cool completely before transferring to freezer-safe containers.
Label and freeze for up to 3 months.
To reheat, thaw overnight in the refrigerator and warm on the stovetop over medium heat until heated through.