For the crust: Combine the butter, flour, sugar, salt and lemon zest in a food processor and pulse until it looks like finely grated Parmigiano. Add the egg yolk and 1 to 2 tablespoons of the ice water. Pulse, pulse, pulse until the mixture comes together. If it seems a bit dry, add a little more water and pulse, pulse, pulse until the mixture comes together into a ball. Dump the whole thing out onto a lightly floured work surface. Knead the mixture only enough to make it a smooth ball, 1 or 2 times. Using a rolling pin or your fingers, roll or press the dough out into an even circle 11 to 12 inches in diameter. Transfer to a cookie sheet lined with plastic wrap and enclose the dough in plastic wrap, then refrigerate for at least 1 hour, or preferably overnight.
For the filling: Preheat the oven to 425 degrees F.
Place the sugar, apple cider, lemon juice and vanilla bean seeds in a 10-inch nonstick ovenproof pan and 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. Cook, swishing the pan around gently to promote even cooking, until the mixture turns light brown, 6 to 7 minutes. Continue to cook the mixture until it becomes a much deeper amber color, another minute or so. 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 pears 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 heat for 20 minutes, then remove from the heat.
Retrieve the chilled pastry and place it on top of the pears. Tuck the pastry in around the edges of the pan. Bake until the dough is golden brown and crispy, 20 to 25 minutes. 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 pieces and garnish with a dollop of sweetened mascarpone.