Preheat the oven to 325 degrees.
Grease a 9x5x3-inch loaf pan with 1 tablespoon of the butter. Dust the pan with the tablespoon of the flour.
Combine 1 cup of the granulated sugar and the lemon juice in a medium-size non-reactive saucepan over medium-high heat. Bring to a boil, stirring to dissolve the sugar, and cook until the mixture reduces by half, about 3 minutes. Remove from the heat and cool.
With an electric mixer fitted with a wire whip, cream the remaining 2 sticks butter in a large mixing bowl until light, fluffy, and smooth. Add the eggs, one at a time, beating well after each addition.
In a medium-size mixing bowl, combine the remaining 2 cups flour, the baking powder, and salt. Add to the butter mixture, about 1/2 cup at a time, beating until all is well blended. Fold in the lemon zest. Pour the mixture into the prepared pan. Bake until the center springs back when touched, about 50 minutes.
Remove from the oven and cool on a wire rack for 10 minutes. Remove the cake from the pan and place it on a wire rack. With a toothpick or wooden skewer, make tiny holes randomly on the top of the cake. Pour half of the sugar-lemon syrup over the cake. Brush the sides of the cake with the remaining syrup.
In a small mixing bowl, combine the confectioners' sugar and water and whisk until smooth. Pour over the top of the cake spread evenly, letting it drip down the sides. Let stand for about 15 minutes.
Slice the cake into 1/2-inch-thick slices. Spread the lemon pastry cream evenly on half of the slices. Place the remaining slices over the lemon pastry cream to form sandwiches. Slice each sandwich in half diagonally and top each with the whipped cream and blueberries.