Peel the apples and cut them in half from top to bottom. Remove the stems and cores with a sharp knife and a melon baller.
Slice the apples crosswise in 1/4-inch thick slices; hold end pieces aside for filling gaps or narrow ends.
Place overlapping slices of apples diagonally down the middle of the tart and continue making diagonal rows on both sides of the first row until the pastry is covered with apple slices.
Sprinkle evenly the 1/2 cup of sugar.
Cut the remaining 4 tablespoons chilled butter into 1/4-inch dice and dot sprinkle over the tart.
Bake for 45 minutes, until the pastry is browned and the edges of the apples start to brown. The sweetened apple juices may burn in the pan but the tart will be fine!
Just before the tart comes out of the oven, heat the apricot jam together with the Cognac and as soon as the tart comes out of the oven, brush it generously over the apples, being careful not to brush the pastry.
Loosen the edges of the tart with a spatula and transfer to a board.
Let cool slightly before serving.