Bring a large pot of salted water to a boil. Cook pasta according to package directions until al dente. Reserve 1 cup of pasta water before draining.

While pasta cooks, heat olive oil and butter in a large skillet over medium-high heat. Add mushrooms and cook until golden brown, about 5-7 minutes. Remove from skillet and set aside.

In the same skillet, sauté the chopped onion until soft, about 3-4 minutes. Add minced garlic and red pepper flakes, cooking for another minute until fragrant.

Pour in the heavy cream and bring to a simmer. Let it cook for 2-3 minutes, stirring occasionally, until it thickens slightly. Reduce heat to low.

Stir in the grated Parmesan cheese until melted. Add the cooked mushrooms and fresh spinach, stirring until the spinach wilts. Season with salt and pepper to taste.

Add the drained pasta to the skillet. Toss everything together to coat the pasta in the sauce. If needed, add a splash of reserved pasta water to reach the desired consistency. Serve immediately.