Spray a 9 X 5 " loaf pan with cooking spray and line it with a double layer of plastic wrap. Leave about 4" overhand on all sides.
Whisk yolks, ⅓ cup sugar and liqueur in the top of a double boiler. Bring water in the bottom of the double boiler to a simmer. Do not let water touch the top of the double boiler. Set double boiler top over the bottom pan and whisk until the yolks thicken and look like soft whipped cream (about 4 minutes). Remove the top and continue to whisk about 3 more minutes until mixture is cool to touch.
Peel, pit and quarter 2 of the peaches. Puree these 2 in processor until smooth. You are shooting for 1 cup of peach puree.
Add the peach puree to the yolk mixture. Whip the cream with ¼ cup of sugar until soft peaks form. Fold cream mixture gently into the peach/yolk mixture.
Fold in the ground praline until evenly mixed. Transfer the mixture to the prepared loaf pan. Fold plastic overhang over to seal the pan. Cover with foil and freeze overnight.
When ready to serve unfold the foil and plastic wrap. Invert semifreddo onto a serving plate. Press chopped praline onto sides of semifreddo. You can cover and return to freezer if not ready to serve yet.
Just before serving peel, pit and slice the other 2 peaches for garnish. Line the slices along the open top of the semifreddo. If you prepare the peach slices ahead of time, add the 1 tablespoon lemon juice to just enough water to cover the peach slices so they don't discolour. Keep them covered until ready to garnish and then drain them, drizzle remaining 3 Tbsps Gran Marnier (optional) and place them.
To serve cut into 1 " slices. Serve with pieces of praline and peaches for garnish.