Preheat oven to 350 degrees. Stir together flour and 3/4 cup brown sugar. Sift twice.
Put egg whites into the bowl of an electric mixer fitted with the whisk attachment. Beat on medium speed until foamy. Add cream of tartar and salt. Raise speed to high, and beat until soft peaks form. Sprinkle half the remaining 3/4 cup brown sugar over egg white mixture, and beat until combined. Sprinkle the remaining brown sugar over egg white mixture, and beat until stiff, glossy peaks form.
Transfer egg white mixture to a large bowl. Fold in flour mixture in 3 batches, folding in lemon zest with last batch.
Spoon batter into an ungreased 10-inch tube pan (not nonstick) with a removable bottom. Run a knife through batter to eliminate air pockets. Bake until cake is golden and springs back when lightly touched, about 45 minutes.
Invert pan onto its legs or over a narrow-neck bottle; let cool 1 hour. Reinvert, and run a knife around sides and tube to loosen; remove sides. Run a long knife along bottom of cake; remove from tube.
Place cake on serving platter. Cut a slit in rind of candied citrus slices; twist slices, and arrange around bottom of cake. Reserve 1 slice for hole in center.
Put creme fraiche into a medium bowl. Whisk until lightened. Transfer a little more than 1/4 cup to a pastry bag fitted with a large rose tip (such as Ateco No. 123). Decoratively pipe creme fraiche around top edge of cake. Place 1 citrus slice over hole; pipe creme fraiche around it. Serve remaining creme fraiche on the side.