Begin by cooking down the berries. In a large pan add all of the berries, lemon juice and sugar. Cook down until the berries just begin to burst and the sauce is syrupy — about 8 minutes.
Pour the mixture into a blender and puree — be careful as it is hot, so use a towel to cover lid and hold down tightly. Strain out any remaining seeds and pulp and cool mixture in the fridge.
Whip egg whites with an electric mixer until you have soft peaks, then add 1/2 cup of the powdered sugar and whip again briefly until you have stiff peaks. Remove 2 cups of the whites and place in a piping bag to make meringue garnish for the semifreddo.
Whip heavy cream with 1/2 cup of powdered sugar until you have stiff peaks. Now combine the mix. In a large bowl with the remaining egg whites slowly fold in the cooled berry puree until you have about 1 1/2 cups left. Reserve this for topping the semifreddo. Once combined, gradually fold in the whipped cream until you have a mousselike texture. Take a glass loaf tin (Pyrex 1 1/2 quart) and line with plastic wrap. Pour the semifreddo mix into the lined vessel, cover with more wrap and refrigerate overnight until just set.
While the ice cream is setting, make the meringue snowcaps. Take a cookie tray and line with parchment or a silpat. Pipe out 2-inch round peaks (like a Hershey’s Kiss), pinching off the pastry bag as you pull up to give it a peak. Bake in a 225-degree F oven for 1 hour, then turn off the heat and leave in the oven to dry out.
To assemble, remove the loaf from the freezer and invert onto a rectangular plate. Remove plastic wrap, then top with meringue snow peaks — you should fit 2 rows. In the center of the rows drizzle plenty of berry puree and finish by dusting the whole dessert with powdered sugar.