To make the crust: In a food processor combine the butter, flour, sugar, salt and lemon zest. Pulse until it looks like finely grated Parmigiano. Add the egg yolk and 1 to 2 tablespoons of the water. Pulse, pulse, pulse until the mixture comes together. If it seems a bit dry add a little more water and pulse, pulse, pulse. The mixture should come together into a ball. Dump the whole thing out onto a clean lightly floured work surface. Knead the mixture 1 or 2 times only to make it a smooth ball. Using a rolling pin or your fingers roll or press the dough out to an even circle about 11 to 12 inches in diameter. Transfer to a cookie sheet lined with plastic wrap and refrigerate for at least 1 hour, or preferably overnight, covered with plastic wrap.
Preheat the oven to 425 degrees F.
To make the filling: While dough is chilling, place the sugar, apple cider, lemon juice, and vanilla bean seeds in a 10-inch nonstick ovenproof pan. Stir to combine. Over high heat bring the mixture to a boil brushing down the sides of the pan occasionally with a pastry brush dipped in water, if necessary. After 6 to 7 minutes the mixture will eventually begin to turn light brown. Swish the pan around gently to promote even cooking. Cook the mixture for another minute or so until the mixture becomes a much deeper amber color. Remove from the heat and stir in the butter, 2 pats at a time. The mixture will bubble up. That is okay, just be VERY CAREFUL not to get any of this on you. When all of the butter has been incorporated, begin to arrange the apples rounded side down in circles. Try to do this neatly and in a pretty way. Remember, the bottom will be the top!
Return the pan to the burner and cook over medium for 20 minutes. Remove from the heat.
Retrieve the chilled pastry from the refrigerator and place it on top of the apples. Tuck the pastry in around the edges of the pan. Bake in the preheated oven for about 20 to 25 minutes or until the dough is golden brown and crispy. Let the tart cool for 10 to 15 minutes. Place a serving platter upside down on top of the pastry and CAREFULLY flip the platter and the pan over. Let the tart fall gently out of the pan.
Slice tart into individual pieces and garnish with a dollop of sweetened mascarpone.