Bring a large pot of salted water to a rolling boil. Add spaghetti and cook according to package instructions until al dente, usually around 8–10 minutes. Reserve 1/2 cup of starchy pasta water, then drain and set aside.

In a large skillet, heat over medium heat and add chopped bacon. Cook for about 5–7 minutes until crispy and golden. Transfer to a plate lined with paper towels to drain.

Pour off excess bacon grease from the skillet, leaving about 1 tablespoon. Add a splash of olive oil if desired. Sauté sliced mushrooms for about 5–7 minutes until browned and tender.

Add minced garlic and cook for an additional minute until fragrant.

Stir in pumpkin puree, dried thyme, smoked paprika, and nutmeg if using. Season with salt and pepper. Cook for 2 minutes.

Pour in heavy cream and grated Parmesan cheese. Stir and let the sauce simmer for 2–3 minutes until thickened.

Add the cooked spaghetti into the skillet, tossing to coat evenly. Add reserved pasta water gradually if the sauce is too thick.

Dish out the pasta into serving bowls, topping with crispy bacon, fresh parsley, and an extra sprinkle of Parmesan cheese.