Roasted Peaches: Sprinkle sugar over bottom of 9-inch (2.5 L) square metal cake pan. Press peach halves in pan to coat the cut side with sugar; place cut-side up. Drizzle with orange liqueur and lemon juice. Roast, uncovered and turning occasionally, in 425°F (220°C) oven until caramelized and tender, 20 to 25 minutes. Set aside to cool. 

Custard: Meanwhile, in medium saucepan, bring 10% cream to simmer over medium-high heat, stirring constantly. Remove from heat. 

In medium bowl, whisk egg yolks and sugar until thick and pale. Gradually pour the hot cream into yolks, whisking constantly. 

Pour mixture into clean medium saucepan and cook over low heat, stirring constantly until thick enough to coat the back of wooden spoon, about 5 minutes. Strain through sieve into clean bowl; refrigerate until cold, 2 to 3 hours. Stir in vanilla and whipping cream. Place plastic wrap directly on surface; refrigerate until cold, 30 minutes.
    
In blender, purée roasted peaches until smooth. Fold into cold custard. Spoon into 9-inch (2.5 L) square metal cake pan; freeze until almost firm, about 3 hours. Break up mixture and purée in blender. Pack into airtight container and freeze 1 hour. 

Transfer to refrigerator 30 minutes before serving. Garnish each serving with peach slice.