Step 1Using a vegetable peeler, remove the peel and white pith from the oranges, lemons and grapefruit. Using a small, sharp knife, trim away all the pith from the peel. Cut the peel into julienne strips (makes about 1 1/2 cups total). Place them in a small saucepan of water.
Step 2Bring the water to a boil. Drain. Rinse the peel under cold water. Repeat this process once.
Step 3Combine the peel, the 1 1/4 cups of water and sugar in the same pan. Simmer gently over medium-low heat until the juices evaporate and the strips become translucent, about 45 minutes. Set aside.
Step 1Heat the oven to 250 degrees. Oil a 3-quart (8-by-4 1/2 -inch) charlotte mold.
Step 2Combine 1 cup of the sugar and the water in a heavy small saucepan. Stir over medium-high heat until the sugar dissolves. Boil without stirring, over medium heat, until the syrup turns a deep amber brown and just begins to smoke, occasionally brushing down the sides of the pan with a wet pastry brush and swirling the pan, about 8 minutes.
Step 3Carefully pour the caramel into the charlotte mold. Using oven mitts, swirl the mold to coat the interior completely with the caramel (the caramel will stick to the pan as it cools). Refrigerate until the caramel is cold and set.
Step 4Using a mandoline, cut the apples crosswise into 1/8 -inch-thick slices. Arrange enough apple slices to cover the bottom of the charlotte mold, overlapping slightly and in concentric circles, to create 1 layer. Sprinkle 2 teaspoons sugar and 1 tablespoon zest confit over the layer of apples. Repeat layering all of the apple slices with the sugar and zest confit, alternating the direction of the concentric circles to increase the stability of the cake, pressing to compact the layers, and creating about 20 layers. Cover with aluminum foil.
Step 5Place the charlotte mold in a deep roasting pan. Fill the pan with enough hot water to come halfway up the sides of the mold. Bake until a skewer inserted near the center of the apples does not meet resistance, about 5 hours. Refrigerate until cold, several hours or overnight.
Step 6Remove the foil. Place the mold in a saute pan of simmering water to melt the caramel coating slightly and help loosen the apple cake, about 3 minutes. Place a platter on top of the mold. Holding the mold with one hand and the platter with the other, invert the mold onto the platter. Remove the mold.
Step 7Cut the apple cake into 12 wedges and transfer to dessert plates. Drizzle with juices from the cake and serve with creme fraiche.