Preheat the oven to 400 degrees. In a food processor, pulse together flour, butter, buttermilk and salt. You’ll come up with a clumpy mixture, which is exactly the consistency you’ll want. By roughly mixing this, the butter will stay in pieces and will melt nicely when baking, creating that yummy flakey crust!
Chill the dough in the freezer while you wait for the oven to preheat and prepare the filling.
Prepare the filling! In a bowl, mix the apricot preserves, mascarpone cheese and sugar. There’s no need to use an electric mixer, remember this is a rustic tart!
Once the oven is preheated, use the remaining ¼ cup of flour to roll out the dough and place onto a well-greased baking sheet. Don’t worry if the dough falls over the edges because you’re going to fold the edges over to make the crust.
In the center place the mixture but not more than ½ inch thick and leave about an inch and a half around the circumference of the tart. It’s okay if you have extra filling (feel free to eat it), you just don’t want to over fill the tart and send the yummy apricots right over the edge.
Gently fold about an inch over the top of the tart, and fold the rest of the dough over in a pinwheel motion layering one fold after the other.
Use the egg white to brush the top of your tart. This will create a nice golden crust and keep the edges of the tart looking shiny.
Bake for 20-25 minutes or until golden brown and let rest for about 10 minutes before serving.