Preheat the oven to 350°F (175°C).
In a large skillet, brown the lean ground beef over medium heat.
Season the beef with salt and pepper, then set it aside to cool.
In a mixing bowl, beat the cream cheese until smooth. Add the eggs one at a time, mixing well after each addition.
Pour the cream cheese mixture over the cooked ground beef and stir to combine.
Transfer the mixture into a small baking dish, pressing it down evenly.
In the same skillet, melt butter and sauté the sliced mushrooms until they are soft and start to turn brown. Season with salt, pepper, seasoned salt, and dried parsley.
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 and the edges are lightly golden.
Optional: Sprinkle bread crumbs or crushed crackers over the top during the last ten minutes of baking.
Remove the casserole from the oven, sprinkle with grated cheese, and let it melt before serving.
