Preheat your skillet. Begin by preheating a 12-inch cast-iron skillet over medium-high heat. Drizzle in the olive oil and wait until it shimmers, signaling it's ready for the beef.

Season the beef. Generously season your beef tenderloin tips with kosher salt and freshly ground black pepper. Once the oil is hot, add the beef to the pan, ensuring not to crowd it.

Sear the beef. Sear the beef tips without moving them for about 1 minute until they develop a beautiful crust. Then, turn and brown all sides for approximately 3 minutes. Remove the beef and set it aside.

Sauté the vegetables. In the same skillet, reduce the heat to medium. Add butter and any drippings left in the pan. Sauté the sliced onions and mushrooms until the mushrooms release their liquid and onions caramelize, about 6-7 minutes.

Add garlic. Introduce minced garlic to the skillet, cooking it for 30-45 seconds until fragrant.

Deglaze the pan. Pour in the beef broth, Worcestershire sauce, and dried thyme. Scrape the bottom of the pan with a wooden spoon to deglaze.

Thicken the gravy. Mix cornstarch with a bit of cold water to create a slurry. Whisk this into the skillet, letting it simmer until the gravy thickens, about 2 minutes.

Combine everything. Return the beef tips to the skillet, along with any juices that have accumulated. Stir to ensure the beef is well-coated in the gravy, cooking for an additional 1-2 minutes. Adjust seasoning to your taste.