Bring a large pot of salted water to a boil. Cook the pasta to al dente according to package directions. Reserve 1/2 cup of the pasta water, then drain and set aside.

Heat the olive oil in a large skillet over medium heat. Add the diced onion and cook for about 5 minutes, stirring occasionally, until softened and nearly translucent. Adjust heat as needed to prevent browning.

Add the mushrooms and cook for about 8–10 minutes, stirring occasionally, until they release their moisture and begin to brown. Add the garlic, salt, black pepper, and red pepper flakes, and cook for 1 minute until fragrant.

Pour in the white wine to deglaze the pan, scraping up any browned bits. Let it simmer for about 2 minutes to cook off the alcohol.

Add the spinach and cook for about 2 minutes, stirring, until wilted.

Add the heavy cream and bring to a gentle simmer (do not boil rapidly). Cook for about 4–5 minutes, until slightly thickened but still loose. Taste and adjust seasoning if needed. (Optional: add a pinch of nutmeg.)

Reduce heat to low. Add the cooked pasta, 1/4 cup of the reserved pasta water, and half of the Romano cheese. Toss to combine.

Add the remaining Romano cheese and continue tossing until the sauce becomes smooth and creamy. Add additional pasta water a little at a time as needed to reach your desired consistency.

Remove from heat. Serve immediately, garnished with additional Romano cheese if desired.