Melt the butter over a medium heat and add the garlic. Let it caramelise for about 5 minutes making sure not to let it burn. Add the sour cream, parmesan, lemon zest and juice, milk (if using) and salt and pepper. Simmer for a minute. Stir through the spinach leaves until wilted.
Remove the sauce from the heat and pour over the cooked pasta. Mix together and serve in bowls topped with a grating of pecorino and a sprinkling of chopped parsley.
Notes
Nutrition information is an estimate and will vary depending on the exact ingredients used.