Preheat oven to 350 degrees. Lightly grease a 13-x-9-x-2-inch baking dish; set aside.

In a large skillet, heat olive oil over medium-low heat. Add onion and cook, stirring occasionally, for 3 to 5 minutes, or until tender. Add mushrooms and garlic. Cook for 2 minutes, or until mushrooms are tender. Add spinach and cook until heated thoroughly. Remove from heat and set aside.

In a large saucepan, melt 5 tablespoons butter over medium heat. Sprinkle flour over melted butter. Cook, stirring constantly, for 3 minutes.

Gradually add milk, whisking to combine. Cook over medium heat, whisking constantly, for approximately 15 minutes, or until mixture is thickened. Remove from heat. Add mozzarella, 1 cup Parmesan, and salt, stirring until melted.

Spoon macaroni into a large bowl. Pour cheese mixture over macaroni. Add spinach mixture; stir. Spoon into prepared baking dish.

Bake for 20 minutes.

Meanwhile, in a small bowl, combine fresh bread crumbs, remaining 3 tablespoons melted butter, and remaining 1/4 cup Parmesan cheese. Remove macaroni from oven and sprinkle with bread-crumb mixture. Return to oven and bake for 5 minutes, or until top is lightly browned.