Before turning on the stove, chop your onions, mince the garlic, slice the mushrooms, and grate the Parmesan. I find that prepping everything ahead makes the cooking process smoother and less stressful.
Bring a large pot of salted water to a boil and cook the pasta according to package instructions until al dente. Drain it and set aside, reserving a cup of the pasta water for later.
Heat olive oil in a large skillet over medium heat. Add onions and garlic, cooking until fragrant, then toss in the mushrooms until they turn golden brown and tender.
Stir in fresh spinach and let it wilt. Pour in heavy cream and broth, stirring to combine. This creates a rich, velvety sauce that clings to the pasta beautifully.
Add the cooked pasta to the skillet and mix thoroughly with the sauce. Gradually stir in Parmesan cheese until melted, adjusting with pasta water if needed for a smoother consistency.
Taste and season with salt, pepper, and red pepper flakes. Serve immediately while hot, garnishing with extra Parmesan or fresh herbs if desired.