Preheat your oven to 350°F (175°C) and line a 10x14 inch baking sheet with parchment paper.

In a large bowl, beat the softened butter and 1/2 cup of sugar until light and creamy. Mix in the egg yolks one at a time.

Stir in the flour, baking powder, milk, and lemon zest until a smooth, thick batter forms. Spread this evenly onto the prepared baking sheet.

In a clean, dry bowl, whip the egg whites until soft peaks form. Gradually add the remaining 3/4 cup sugar, whipping until stiff, glossy peaks form.

Gently spread the meringue over the cake batter layer on the baking sheet and sprinkle the sliced almonds on top.

Bake for 30 to 35 minutes until the meringue is golden brown and crisp. Allow the cake to cool completely in the pan.

Whip the heavy cream until it holds soft peaks, then gently fold in the thick vanilla custard until combined.

Carefully cut the cooled cake in half vertically to create two equal rectangles.

Place one cake half on a serving platter, meringue side up. Spread the custard cream mixture over it and scatter the fresh blueberries on top.

Place the second cake half on top, meringue side up, to form a sandwich. Chill for 1 hour before slicing to allow the filling to set.