01 - Bring a large pot of salted water to a boil. Cook pasta until al dente according to package instructions. Reserve 1/2 cup of pasta water, then drain.
02 - Heat olive oil in a large skillet over medium heat. Add garlic and shallot, sauté for 2 to 3 minutes until fragrant and softened, avoiding browning.
03 - Add baby spinach to the skillet, stirring until just wilted, about 2 to 3 minutes.
04 - Reduce heat to low. Stir in heavy cream and whole milk, simmer gently for 2 to 3 minutes to combine flavors.
05 - Add grated Parmesan, lemon zest, lemon juice, salt, black pepper, and red pepper flakes if using. Stir until cheese melts and sauce is creamy.
06 - Add drained pasta to skillet. Toss to coat evenly, adding reserved pasta water gradually until sauce clings smoothly to noodles.
07 - Taste and adjust salt and pepper if necessary. Serve immediately garnished with extra Parmesan, black pepper, and lemon zest.