In a large skillet over high heat, combine butter, sugar, and lemon juice. Stir until mixture forms a nice syrup. You may need to add more butter if mixture is too dry. Add lemon zest and nectarines and stir until just soft. If you have it on hand, a splash of Cognac never hurt anyone and it'll make a nice flame (if you're cooking with gas). Spoon mixture into individual serving cups and top with cold creme fraiche or nice vanilla bean ice cream.