01 - Bring a large pot of salted water to a boil. Cook the pasta according to package directions until al dente. Reserve 1/2 cup of pasta cooking water, then drain.
02 - In a large skillet over medium heat, melt butter with olive oil. Add shallot if using; cook 2 minutes until softened. Stir in garlic and cook 1 minute until fragrant.
03 - Add spinach to the skillet and cook, stirring, until wilted, about 2 minutes.
04 - Pour in heavy cream, bring to a gentle simmer, then stir in Parmesan cheese until melted and sauce is smooth.
05 - Add cooked pasta to the skillet and toss to coat. Gradually add reserved pasta water, tablespoon by tablespoon, until desired sauce consistency is reached.
06 - Stir in lemon zest and lemon juice. Season with salt, pepper, and red pepper flakes to taste.
07 - Serve immediately, garnished with extra Parmesan and lemon zest.