Season the bite-sized pieces of beef steak with salt and pepper. Heat a skillet over medium-high heat and add olive oil. Sear the beef in batches until browned on all sides, about 3-4 minutes per side. Once browned, transfer the beef to your slow cooker.

In the same skillet, add the finely sliced onion and minced garlic, sautéing for 3-5 minutes until soft and translucent. Add sliced mushrooms and continue cooking for an additional 4-5 minutes.

Pour in the brandy or cognac (if using) to deglaze the skillet, scraping up any browned bits. After a minute of simmering, transfer the onion and mushroom mixture to the slow cooker. Add beef broth, Dijon mustard, and Worcestershire sauce, mixing well.

Cover your slow cooker and set it to cook on low for 6-8 hours or high for 3-4 hours until the beef is fork-tender.

Once cooking is complete, stir in the heavy cream and check seasoning. For a thicker consistency, mix cornstarch with water and stir into casserole; cook on high for an additional 15-20 minutes.

Garnish your casserole with fresh chopped parsley and serve with creamy mashed potatoes, rice, or a refreshing salad.