Crust: In a food processor combine the butter, flour, sugar, salt and lemon zest. Pulse until it looks like finely grated Parmesan. 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. Remove the dough to 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 the crust to a cookie sheet lined with plastic wrap and refrigerate it for at least 1 hour, or preferably overnight, covered with plastic wrap.
Preheat the oven to 425 degrees F.
Filling: While dough is chilling, put the sugar, apple cider, lemon juice, and vanilla bean seeds in a 10-inch nonstick ovenproof pan. Stir to combine. Bring the mixture to a boil over high heat, 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 it 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 let any of this touch your skin. 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 become the top!
Return the pan to the burner and cook over medium heat for 20 minutes. Remove the pan from the heat.
Retrieve the chilled pastry from the refrigerator and arrange it on top of the apples. Tuck the pastry in around the edges of the pan. Bake in the preheated oven until the dough is golden brown and crispy, about 20 to 25 minutes. Remove from the oven and let it cool for 10 to 15 minutes. Put a serving platter, upside down, on top of the pastry and CAREFULLY flip the platter and the pan over. Let the tarte fall gently out of the pan.
Slice the tarte into individual pieces and garnish with a dollop of sweetened mascarpone.