Cook the penne pasta in a large pot of boiling salted water until al dente, then drain and set aside.
Heat olive oil in a sauté pan over medium heat, then add sliced mushrooms and cook, stirring occasionally, until they release their moisture and turn golden brown, about 5-7 minutes.
Add the minced garlic to the mushrooms and cook just until aromatic, about 30 seconds. Then, stir in the chopped spinach and cook until wilted, about 2-3 minutes.
Pour in the creamy white sauce into the sauté pan and stir to combine the mushrooms and spinach evenly, heating untileverything is warmed through.
Transfer the cooked pasta into a large mixing bowl and pour the mushroom, spinach, and sauce mixture over it. Toss gently to coat all the pasta evenly with the sauce.
Preheat your oven to 375°F (190°C). Spread the coated pasta into a baking dish, then sprinkle the shredded mozzarella evenly across the top.
Bake in the preheated oven until the cheese is golden and bubbling, about 20-25 minutes.
Remove the pasta bake from the oven and let it rest for a few minutes, allowing the flavors to settle and the cheese to firm up slightly.
Serve warm, scooping out generous portions that showcase the ooey-gooey cheese, tender pasta, and savory mushroom-spinach mixture.
