For the Cheesecake

1. Preheat oven to 425°F.  Line a 9-inch springform pan with two 15×15-inch pieces of parchment paper, extending over edges of pan 2 to 3 inches.  Place prepared pan on a large rimmed baking sheet.
2. For filling, in a large bowl beat together sugar, lemon zest and lime zest until the zest is evenly distributed througout.
3. Add cream cheese and mascarpone; beat on medium until smooth and sugar has disolved.
4. Beat in eggs, one at a time, until combined. 
5. Add cream, liqueur, salt and vanilla.  Beat on medium until combined, scraping bowl as needed (mixture will be thin)
6. Pour filling into prepared pan.  Tap on counter a few times to remove any air bubbles.
7. Bake 50-55 minutes or until the center is still jibbly and cheesecake is set an inch from the edge when shaken.  Surface should be dark brown.
8. Cool in pan on a wire rack 15 minutes.
9. Remove the sides of the pan, the cool completely on a wire rack.
10. Gently peel off parchment. Serve at room temperature or cover and chill up to 3 days.
11. Let stand at room temperature 30 minutes before serving. Top with salt flakes (if using), and garnish with Candied Citrus Fruit (optional).

For the Candied Citrus Fruit

1. In a Dutch oven, combine sugar and water; bring to a boil. Add citrus slices; reduce heat to medium.
2. Cook until slices are translucent, about 20 minutes, turning occasionally.
3. Reduce heat; simmer until slices are tender but still intact, about 10 minutes, turning occasionally.
4. Using a slotted spoon or tongs, remove slices to a wire rack or a parchment-lined baking pan. Let stand at room temperature overnight to dry. (Save syrup for another use.)
5. If desired, cut slices in half; use to decorate a cake, pie or other dessert, or to garnish drinks.