01 - Bring a large pot of salted water to a boil. Cook the pasta according to package instructions until al dente. Reserve 1/2 cup of pasta cooking water, then drain the pasta.
02 - While the pasta cooks, heat olive oil in a large skillet over medium heat. Add garlic and shallot; sauté for 2–3 minutes until fragrant and soft, but not browned.
03 - Add the spinach and cook, stirring, until just wilted, about 2–3 minutes.
04 - Reduce heat to low. Pour in the heavy cream and milk, stirring to combine. Simmer gently for 2–3 minutes.
05 - Add grated Parmesan, lemon zest, lemon juice, salt, black pepper, and red pepper flakes (if using). Stir until the cheese melts and the sauce becomes creamy.
06 - Add the drained pasta to the skillet. Toss well to coat, adding reserved pasta water a little at a time until the sauce clings smoothly to the noodles.
07 - Taste and adjust seasoning if needed. Serve immediately, garnished with extra Parmesan, black pepper, and lemon zest.