Make the blueberry puree: Place the 3 cups blueberries, water, sugar, lemon zest and juice in a small saucepan.  Bring to a boil and stir. Reduce heat to medium low and simmer until the mixture is slightly thickened, 15 to 20 minutes. Remove from the heat and stir in the vanilla, cinnamon and nutmeg.

Prepare the pan: Line a 9 inch X 5 inch loaf pan with plastic wrap that covers the bottom and sides and overhangs with enough plastic to completely cover the semifreddo on the top.

Whip the cream: Using an electric mixer, beat the heavy cream in a large bowl until soft peaks form.  Cover and refrigerate until ready to use.

Prepare the custard:  Prepare a hot water bath by setting a heat-proof bowl over a pan of water. Bring to a simmer. Be sure the bottom of the bowl does not touch the water or the egg yolks will cook. In the large bowl, whisk together the egg yolks and sugar. (You want the bowl large enough to accommodate the whipped cream as well.) Set the bowl over the simmering water and whisk constantly until the mixture is thick and an instant-read thermometer reads 170 degrees Fahrenheit. Remove the bowl from over the pan.  Using a electric mixture, beat the egg yolk mixture until thick and pale, about 5 minutes.  The mixture should be about double in size.

Assemble the semifreddo: Gently fold the whipped cream into the egg yolk mixture.  Grab your prepared pan and blueberry mixture.  Reserve about ½ cup of the blueberry mixture for topping. Gently add about 1/3 of the semifreddo mixture to the pan.  Give it a good tap on the work surface to remove any air pockets. Add about 1/3 of the remaining blueberry mixture over and gently swirl into the semifreddo using a wooden skewer or knife tip.  Add another 1/3 of the semifreddo mixture followed by the blueberry mixture. Swirl. Finish with a final round of the semifreddo and blueberry mixtures. Give it a final tap on the work surface.  Fold the plastic overhang tightly over the top of the pan to cover.  Please in the freezer and freeze until firm, at least 8 hours or overnight.

To serve: Remove the semifreddo from the freezer and allow it to sit at room temperature for about 15 minutes.  Uncover and holding the pan in one hand, flip the pan onto a serving platter.  Remove the plastic wrap.  Slice the semifreddo into 1 inch thick slices and top with the reserved blueberry sauce and a few mint leaves.  Serve!