Heat the slow cooker on the high setting. To the slow cooker, add the beef stock and mushrooms and cover.
Heat the butter in a large sauté pan, over medium heat. Add the onions and garlic to the pan and sauté until the onion is translucent and the garlic is fragrant. Add them to the slow cooker.
Sprinkle the sirloin with salt, add it to the pan and sear on both sides, about 2 minutes.
Add the steak, heavy cream, sour cream, beef bouillon, Dijon, parsley, onion powder, garlic powder, and oregano to the slow cooker. Cover and cook on high for 6 hours.
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