Pound meat with a mallet to ½ inch thickness. Season with salt & pepper.
Heat olive oil in a Dutch oven over medium-high heat and brown steaks on each side (add more olive oil if needed). Remove and set aside.
Add mushrooms, onions, and butter to Dutch oven. Cook until tender, about 5 minutes then add the steaks back to the pan.
Combine beef broth, thyme, and Worcestershire sauce, and pour into pan. Cover and bake in oven for 2½-3 hours, or until fork-tender. Remove steaks from liquid and set aside.
Make a slurry by combining cornstarch and water until smooth. Bring liquid to a boil and whisk cornstarch mixture in, a little bit at a time, until it reaches desired consistency.
Notes
Brown steaks only slightly, then cook low and slow in the dutch oven. Refrigerate leftovers in a covered container for up to 4 days and freeze portions for up to 4 months.