Make the curd:
In the bowl portion of a double boiler, whisk together the eggs, yolks, sugar, orange and lemon juice and zest. Place the bowl over simmering water and whisk continuously, until thickened, about 10-15 minutes.
Whisk in the butter, little by little and continue whisking until curd is smooth and glossy. (if desired, you can strain the curd through a fine sieve), or not. Cover the curd with plastic wrap, pressing the wrap onto the surface of the curd so a skin does not form and allow the curd to cool completely. When cool, fold the whipped cream into the curd to lighten.
Shortbread Crust and Merinque
Make the crust:
In a food processor, pulse flour, sugar, zest and salt until well mixed. Add the butter and process, stopping to pulse occasionally, until the dough is crumbly, and just starts to come together into clumps. This will take a minute or two of processing.
Dump the clumpy dough into a sprayed tart pan and press to fill the pan in an even layer. Make sure the edges are even and use the bottom of a glass to press the dough into the corners of the pan. Use the bottom of the glass to smooth the surface of the dough out.
Using a fork, prick the bottom of the tart (to keep it from puffing up when it bakes), cover in plastic wrap and put the tart in the freezer for about 30 min.
Bake at 425 for about 12 minutes, until golden brown
Make the meringue and assemble:
When tart is cooled, make the meringue. Place the egg whites and cream of tartar in a mixing bowl and beat on high with a whisk attachment until the egg whites form gentle peaks, a couple of minutes.
Combine the sugar and water in a small pan over high heat. Cook, stirring occasionally until the sugar is melted and the syrup reaches 240F on a candy thermometer (soft-ball stage).
With the mixer on high, slowly drizzle the hot syrup into the egg white mixture and continue to whip until mixture is stiff and glossy.
Pile the meringue onto the cooled tart and make fun peaks using the back of a spoon. Toast the top with a torch, under a preheated broiler for 30 seconds to a minute.