Preheat your oven to 350°F (175°C) and grease a 9x13-inch casserole dish with butter or cooking spray.

In a large skillet over medium heat, cook the lean ground beef for about 5–7 minutes until thoroughly browned. Season with salt and pepper, then set aside.

Blend the softened cream cheese in a food processor until smooth. Add the eggs one by one, continuing to blend until well combined. Fold in the browned ground beef.

Melt about 2 tablespoons of butter in the same skillet over medium heat. Sauté the sliced mushrooms for about 5–7 minutes until golden brown. Stir in salt, pepper, and dried parsley.

In the greased casserole dish, layer the ground beef and cream cheese mixture as the base. Top it with the sautéed mushrooms and sprinkle with grated cheese if desired.

Bake in the preheated oven for 20–25 minutes until the edges turn golden brown and the center is set.

Let the Hamburger Mushroom Bake rest for 5–10 minutes before serving.