Stir in heavy cream, miso, Dijon, and 1/2 cup reserved pasta cooking liquid ... as needed to thin sauce. Garnish with Parmesan, and serve with lemon wedges.
Melt the butter in a deep, heavy 12-inch skillet and stir in the cream and lemon juice ... (Add more pasta cooking liquid 1 tablespoon at a time, if necessary, to thin the sauce.) ...