Slice oranges thinly.
Boil water and sugar. Steep oranges in cold syrup for an hour.
Make Grand Marnier syrup with remaining sugar, water, and Grand marnier. Cool.
Slice genoise into 2 layers.
Line a 9 inch springform pan with wax paper and arrange sliced oranges on paper. Save a few slices. Spoon 1/2 mousse into pan.
Top with a genoise half. Brush with 1/2 of syrup. Press orange slices into sides of pan, rounded sides down.
Spread remaining mousse over cake.
Top with other genoise. Brush with syrup. Refrigerate 4 hours.
Invert cake on platter. Heat and release pan sides. Peel off wax paper. Brush with top with melted apricot, and decorate with chocolate.