Preheat the oven to 350°F.
In a large skillet over medium heat, cook the lean ground beef until fully browned, breaking it apart as it cooks.
Season the cooked ground beef with salt and black pepper.
Drain any excess grease, if necessary, and set the beef aside to cool slightly.
In a mixing bowl, beat the softened cream cheese until smooth.
Add the eggs one at a time, mixing well after each addition until the mixture is fully combined.
Pour the cream cheese and egg mixture over the cooled ground beef.
Stir until the beef is evenly coated.
Transfer the beef mixture to a small baking dish and press it gently into an even layer.
In the same skillet, melt the butter over medium heat.
Add the sliced mushrooms and season them with salt, black pepper, seasoned salt, and dried parsley.
Cook the mushrooms until they are soft and begin to brown.
Spread the sautéed mushrooms evenly over the beef mixture in the baking dish.
Bake for 20 to 25 minutes, or until the casserole is set.
If using bread crumbs or crushed crackers, sprinkle them over the top during the last 10 minutes of baking.
Sprinkle the shredded or grated cheese over the top and allow it to melt before removing the casserole from the oven.
Let the casserole rest for a few minutes before serving.