Step 1: Start by preheating your oven to 350 degrees Fahrenheit (175 degrees Celsius).
Step 2: In a large pan over medium heat, heat the olive oil. Season the bite-sized beef pieces with salt and pepper, searing them until they develop a golden brown crust on both sides. This step enhances the flavor and seals in the juices. Set the seared meat aside.
Step 3: In the same skillet, add the finely chopped onion and sliced mushrooms. Sauté them for approximately 5 to 7 minutes until they become soft, and the mushrooms release their moisture. Stir in the minced garlic during the last minute of cooking.
Step 4: Sprinkle the flour over the sautéed mushrooms and onions, making sure to mix it thoroughly for even distribution. Gradually pour in the beef broth while stirring continuously to prevent lumps. Let the mixture simmer for 2-3 minutes to thicken.
Step 5: Add the dried thyme and pour in the heavy cream. Simmer for an additional 2-3 minutes, stirring occasionally until the sauce reaches the desired consistency and is seasoned to taste with salt and pepper.
Step 6: Return the seared beef pieces to the pan, ensuring they are evenly coated with the creamy mushroom sauce. Let the mixture simmer at low heat for another 2-3 minutes to allow the flavors to meld.
Step 7: Grease a casserole dish with butter and evenly transfer the beef and mushroom mixture into it.
Step 8: Place the casserole dish in the preheated oven and bake for approximately 25 to 30 minutes, or until the top is lightly browned, and the casserole is bubbling.
Step 9: Once done, remove it from the oven and allow it to cool for a bit. For the best experience, serve the Savory Mushroom Beef Casserole over cooked egg noodles or rice to fully enjoy the rich sauce permeating each and every mouthful.