Heat vegetable oil in a large skillet over medium-high heat.
Add beef tips in batches and brown on all sides, about 3-4 minutes per batch. Remove beef and set aside.
In the same skillet, add diced onion and cook until softened, about 5 minutes.
Add minced garlic and sliced mushrooms, cooking until mushrooms release their moisture and begin to brown, about 6-7 minutes.
Sprinkle flour over the vegetables and stir well to coat. Cook for 2 minutes to remove raw flour taste.
Gradually whisk in beef broth and water, stirring constantly to avoid lumps.
Add Worcestershire sauce, dried thyme, salt, and black pepper. Stir to combine.
Return browned beef tips to the skillet, bring mixture to a simmer.
Reduce heat to low, cover, and cook for 1 hour or until beef is tender and gravy has thickened.
Stir in unsalted butter until melted and incorporated for a glossy finish.
Allow the beef tips and gravy to cool completely before portioning into freezer-safe containers.
Label and freeze for up to 3 months. To reheat, thaw overnight in the refrigerator and warm on the stove over medium heat until heated through.