Preheat oven to 325°F or 160°C.
Rub the beef cut with 1 tbsp olive oil and season with sea salt and black pepper all over.
Turn on the stove. In a skillet, add 1 tbsp olive oil and the beef. Sear on high heat on every side for 1 minute each. Turn off the heat. Don't clean the skillet yet, set it aside and use it later for gravy.
Place the mushrooms in a baking tray. Add garlic, thyme, and ½ tbsp of olive oil around the mushrooms. Place the beef on top.
Roast the beef for 40 minutes on the middle shelf until the temperature inside the beef is 135°F or 57°C (medium rare to medium).
Remove from the oven and transfer the beef and mushrooms to a different plate. Cover the beef with aluminum foil and let rest for at least 15 minutes.
Transfer the rest of the beef bits and stock from the baking tray to a skillet that we use earlier (thyme removed).
In a cup add beef stock or water with 1 tbsp cornstarch. Stir to combine.
Turn the stove on medium heat and add the cornstarch mixture. Keep stirring until the gravy thickens. Turn off the heat. Add salt and pepper if necessary.
The beef will keep warm for about several hours. If you have guests, slice the beef on the table after the guests have arrived and serve immediately.
This recipe will give medium rare to medium beef. Use this information for your roast: