Preparation

• Position a rack in the center of the oven, and heat the broiler on high. Bring a large pot of well-salted water to a boil.
• In a large, deep ovenproof skillet over medium-high heat, melt the butter. Add the onion, season with 1/2 tsp. salt, and cook, stirring occasionally, until starting to brown, about 3 minutes. Add the mushrooms, and cook until they’re browned and
  the liquid is almost all evaporated, about 8 minutes.
• Add the spinach to the skillet, and toss until wilted, about 1 minute. Add the cream and 1 tsp. salt, and bring to a simmer.
  Remove from the heat, and stir in 3 cups of the cheese.
• Meanwhile, cook the pasta according to package directions until al dente. Drain the pasta well, and return to the pot. add the
  sauce and stir to combine. Season to taste with salt and pepper. Transfer the pasta mixture back to the skillet, and sprinkle
  with the remaining cheese.
• Broil until the top is golden-brown in spots, 5 to 7 minutes. Let rest for 5 minutes for the sauce to thicken before serving.