Preheat your oven to 375°F (190°C) and lightly grease a 9x13-inch baking dish.

Cook the penne pasta in a large pot of salted boiling water according to package directions until al dente. Drain and set aside.

In a large skillet, melt the butter and olive oil over medium-high heat. Add the sliced mushrooms and sauté for about 5 minutes until browned and tender.

Add the minced garlic and baby spinach to the skillet, cooking for another 2 minutes until the spinach is wilted. Remove the mixture from the skillet and set aside.

In the same skillet, melt another tablespoon of butter over medium heat. Whisk in the flour and cook for 1 minute to remove the raw flour taste.

Slowly pour in the warm milk and vegetable broth while whisking constantly to prevent lumps. Bring the mixture to a simmer and cook until thickened, about 4 to 5 minutes.

Stir in 1 cup of mozzarella cheese, the parmesan cheese, salt, black pepper, and nutmeg until the cheeses are completely melted and the sauce is smooth.

Add the cooked pasta and the mushroom-spinach mixture into the creamy cheese sauce, tossing gently until everything is evenly coated.

Transfer the pasta mixture into the prepared baking dish and sprinkle the remaining 1/2 cup of mozzarella cheese evenly over the top.

Bake uncovered for 20-25 minutes, or until the cheese is melted, bubbly, and lightly golden brown on top. Let cool for 5 minutes before serving.