Combine beef, breadcrumbs, milk, onion, Worcestershire, garlic powder, salt, and pepper in a bowl. Mix gently.
Shape into 6 oval patties, about 3/4 inch thick. Chill 10 minutes to help them hold.
Heat olive oil in a skillet over medium-high. Brown patties 2–3 minutes per side. Transfer to a plate.
Whisk mushroom soup, beef broth, ketchup, Dijon, and thyme until smooth.
Scatter mushrooms in the slow cooker. Nestle browned patties on top and pour the gravy mixture over them.
Cover and cook on Low for 5 hours, until the patties are tender and cooked through.
Whisk cornstarch with water. Stir into the hot cooking liquid and simmer until thickened, or cook on High 10–15 minutes.
Spoon gravy over the steaks and sprinkle with parsley. Serve hot with mashed potatoes or egg noodles.