Drain the raspberries in a strainer set over a bowl. Preheat the oven to 400 degrees . Using your fingers, combine the butter, 1/2 cup of the sugar and 1 1/2 cups of the flour and blend together until coarse crumbs form. Pour the mixture into an ungreased nonstick 9-inch springform or cake pan and press down evenly to form a crust. Bake the shortbread crust until lightly golden, about 20 minutes.
Remove the shortbread from the oven and reduce the temperature to 325 degrees . Scatter the drained raspberries evenly over the baked shortbread.
Beat the eggs with the remaining 3/4 cup of sugar until thick and pale yellow. Whisk in the lemon zest, the lemon juice and the remaining 1/4 cup of flour. Pour the lemon cream over the raspberries and bake the tart until the custard is set, about 30 minutes. Let cool, then dust with confectioners sugar.