Preheat and prep. Heat your oven to 375°F (190°C). Lightly oil a 9x13-inch baking dish or similar casserole.

Cook the pasta. Bring a large pot of salted water to a boil. Add the penne and cook until just shy of al dente (about 1–2 minutes less than the package suggests). Drain and set aside.

Sauté the aromatics. In a large skillet, warm the olive oil over medium heat. Add the onion and a pinch of salt. Cook until softened and lightly golden, about 5–6 minutes. Stir in the garlic and cook for 30 seconds, just until fragrant.

Cook the mushrooms. Add the sliced mushrooms to the skillet. Season with a little salt and pepper. Cook, stirring occasionally, until they release their liquid and turn browned and tender, 7–9 minutes. Let most of the moisture evaporate so the bake won’t be watery.

Wilt the spinach. Add the spinach in batches, tossing until it wilts down. If using baby spinach, this takes 1–2 minutes. Taste and adjust salt and pepper.

Build the sauce. Pour in the marinara sauce. Add the Italian seasoning and red pepper flakes if using. Simmer for 2–3 minutes to let the flavors meld. If the sauce seems too thick, splash in a little water. If too thin, simmer another minute.

Combine with pasta. In a large bowl (or the pasta pot), mix the cooked penne with the mushroom-spinach sauce. Dollop the ricotta in small spoonfuls and gently fold it through so you get creamy pockets throughout.

Assemble the bake. Transfer the mixture to your prepared baking dish. Sprinkle evenly with the mozzarella, then the Parmesan.

Bake. Place on the center rack and bake for 18–22 minutes, until the cheese is melted and the edges are bubbling. For extra browning, broil for 1–2 minutes at the end. Watch closely to avoid burning.

Rest and serve. Let the casserole rest for 5–10 minutes so it sets and slices cleanly. Garnish with fresh basil or parsley and serve warm.