Pound meat with a mallet to ½ inch thickness. Season with salt & pepper.
Heat olive oil in a dutch oven, and brown steaks on each side (add more olive oil if needed). Remove and set aside.
Add mushrooms, onions, and butter to hot 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.