1. To make the citrus curd: Zest the lemons, blood oranges, and lime; set the zests aside. Squeeze the citrus juice (it should yield 1 cup or 240 mL) and combine the juices with the sugar, butter, and salt in a medium saucepan. Bring the mixture to a boil over medium heat, stirring occasionally.

2. Meanwhile, whisk the eggs and egg yolks in a medium heat-proof bowl until combined. While whisking rapidly with one hand, pour the hot citrus-sugar mixture slowly in the eggs. Return the egg mixture to the saucepan and cook over medium heat, stirring constantly with a heat-proof spatula, until lightly thickened, 2 to 3 minutes. Do not boil or the eggs will curdle. The curd should coat the back of the spatula. It will thicken further as it cools.

3. Immediately transfer the citrus curd to a clean heat-proof bowl (otherwise it will continue to cook in the hot pan and could scramble) – I prefer to pour the curd through a fine-meshed sieve to strain and ensure it is extra smooth. Mix the gelatin and water and mix in. Stir the reserved citrus zests into the warm curd. Continue stirring for 1 minute to stop the cooking and then let continue to cool at room temperature for 30 minutes.

4. Transfer the curd into the cooled tart shell, and spread into an even layer. Refrigerate the tart, covered, for at least 2 hours, or up to 24 hours, until it is set.