Season the beef steak pieces generously with salt and pepper.

In a skillet over medium-high heat, heat the olive oil. Sear the beef in batches until browned on all sides but not cooked through. Transfer the browned beef to the slow cooker.

In the same skillet, add a bit more oil if needed. Sauté the onion and garlic until soft and translucent, then add the mushrooms and cook until they're browned and their moisture has evaporated.

Add the brandy or cognac to the skillet to deglaze the pan, scraping up any browned bits from the bottom. (If not using alcohol, skip to the next step.)

Transfer the onion, garlic, and mushrooms mixture to the slow cooker over the beef. Stir in the beef broth, Dijon mustard, and Worcestershire sauce.

Cover and cook on low for 6-8 hours, or on high for 3-4 hours, until the beef is tender.

Once the beef is tender, stir in the heavy cream. Adjust the seasoning with more salt and pepper if needed. If the sauce is too thin, you can thicken it by mixing a tablespoon of cornstarch with 2 tablespoons of water and stirring it into the casserole. Let it cook for an additional 15-20 minutes on high if using the cornstarch slurry.

Serve the Steak Diane Casserole garnished with chopped fresh parsley. This dish goes well with mashed potatoes, rice, or a simple green salad.