Cover each disk with apple slices arranged in a rosette and place in the oven for 10-15 minutes until the pastry is puffed and baked.
Let reduce by half over low heat until the sauce is thick enough to coat a spoon; gradually whisk in some small pieces of cold butter.
Cook the foie gras slices in a very hot un-greased skillet: sear for 30 seconds on each side on high heat and 3 minutes on medium heat; sprinkle lightly with fleur de sel; place two slices on each individual “Tarte Tatin”; spoon some of the sauce over top and serve immediately.