Cook pasta al dente according to the package.
While the pasta is cooking add the butter and garlic to a skillet.
When the butter is melted, add in the heavy cream, lemon juice, and lemon zest, and simmer for 10 minutes.
Add ¼ cup of the pasta water to the lemon cream sauce and stir.
Add the pasta right from the water into the skillet with the cream sauce.
Add the red pepper flakes and pepper.
Toss the pasta so that it’s completely covered.
Add a bit more pasta water if needed.
Remove the pasta from the heat, and add parmesan.
Add more black pepper if needed.
Serve with lemon slices for garnish.