Pat the steak dry, then season both sides with salt and pepper.
Heat oil in a skillet over medium-high. Sear steak 2–3 minutes per side.
Place onions and mushrooms in the slow cooker. Scatter the garlic over top.
Whisk broth, condensed soup, Worcestershire, Dijon, thyme, and onion powder until smooth.
Set the seared steak on the vegetables. Pour the gravy mixture over everything.
Cover and cook on Low 7–8 hours or High 3–4 hours, until fork-tender.
Stir cornstarch and water to make a slurry. Stir it into the cooking liquid.
Switch to High and cook 10–15 minutes, until the gravy thickens.
Slice or shred the steak and return it to the gravy. Garnish with parsley and serve.