Mascarpone is a soft, sweet cheese. Not sugar-sweet, but mild and milky with just the tiniest bit of tang. The Meyer lemon has that same quality: sweeter than a regular lemon but still tart. Together, they make a great sauce, lighter than a typical cream sauce but still silky and rich.
The hazelnuts in this sauce were a last-minute addition. I made this pasta once without them, and even tested a few bites with the nuts sprinkled on top before I dumped the whole lot into the pan. They really elevate this dish to something special, by adding another texture and flavor.
I didn’t bother precooking the spinach. Instead, I threw it in with the warm pasta and sauce, adding a tiny bit of pasta water to keep the mixture loose while I continued to heat it over low. The spinach wilts in no time. In fact, the entire dish doesn’t take more than 15 minutes to pull together.
Look for mascarpone cheese in the deli cheese section. Meyer lemons, usually grown in California, are becoming more common in well-stocked supermarkets. Even Costco sometimes has them. If you can’t find them, substitute a large lemon.