Bring a large pot of salted water to a boil.
Add the pasta and cook according to package directions for al dente. (It will continue to cook in the sauce to make it more tender.)
While the pasta is cooking, heat a large skillet over low heat.
Add oil, garlic, and crushed red pepper. Cook until the oil sizzles (about 1-2 minutes).
Add lemon juice, half-and-half, and about 1/4 cup of the pasta water to the garlic oil mixture.
Raise the heat to med-low to med heat until the sauce begins to bubble.
When the pasta is done, drain it and add the pasta, lemon zest, and 1 cup of the Parmesan cheese to the cream sauce.
Toss for a minute or two.
Add sliced chicken and spinach, and toss until well distributed.
Top with remaining cheese and serve.