Zest the orange then squeeze the juice out of a blood orange and a red grapefruit and measure out ¼ cup of juice from each. Strain out the seeds and put the juice into a medium size mixing bowl along with the fresh squeezed lemon juice.
Add one can of sweetened condensed milk.
Put 2 small bowls on the work surface and separate the eggs. Begin by cracking the egg in half and transferring the egg from shell to shell and letting the egg white fall into one of the bowls. It should only take a few times transferring the egg from shell to shell to effectively separate the yolk from the white. Put the egg yolk into the other bowl. Continue separating the rest of the eggs.
Put the egg yolks into the bowl with the citrus juice and sweetened condensed milk.
Mix it on medium speed with an electric mixer for one minute until it is combined and smooth. The mixture will be runny.
Pour it into the prepared tart shell and bake it in the oven for 20-25 minutes until it just begins to set before adding the meringue topping.
While the tart is in the oven, make the meringue.
Put the egg white into the mixing bowl of a stand mixer and with the whisk attachment whisk it on medium-high speed for about 2-3 minutes, until soft peaks form and it becomes very frothy.
While the mixer is still going, SLOWLY begin to add the granulated sugar, only about 1-2 tablespoons at a time, over about a 6-minute span.
After 6 minutes, scrape down the sides to make sure all of the sugar is incorporated and mix it again for another minute or so.
The meringue topping should be shiny and glossy and stiff peaks should easily form and stay formed.
After the tart has baked for about 20-25 minutes and it is just beginning to set, remove it from the oven and add the meringue topping, spreading it all the way to the edges.
With the back of a spatula, dab the top of the meringue to make small, decorative peaks.
Place the tart back in the oven for another 5-8 minutes.
After 5 minutes turn the broil function on on the oven and broil for 1-2 minutes, watching it constantly so as not to burn it!
As soon as the tops of the meringue peaks are browned, remove it from the oven.
Let it cool to room temperature, then place it in the fridge for 2 hours before serving. * Enjoy!
Notes
This citrus tart should be served the same day it is made. If you serve it the next day you will notice that the meringue has wept and it will be slightly watery.