In a medium saucepan, combine fresh blueberries, water, 4 teaspoons of sugar, lemon zest, and lemon juice. Stir gently and bring to a simmer over medium heat for 15-20 minutes until thickened.

Line a 9x5 inch loaf pan with plastic wrap, ensuring the wrap hangs over the edges.

In a large mixing bowl, beat 2 cups of heavy cream on medium speed until soft peaks form. Chill.

Set up a double boiler. In a heatproof bowl, whisk together 5 egg yolks and 1 cup of sugar over simmering water until thickened.

Gently fold the whipped cream into the egg yolk mixture with a rubber spatula.

Spoon one-third of the semifreddo mixture into your loaf pan, layer with blueberry puree, and repeat twice more, finishing with semifreddo.

Cover with plastic wrap and freeze for at least 8 hours or overnight.

Let the semifreddo sit at room temperature for about 15 minutes before slicing and serving garnished with blueberry sauce and mint.