SEMIFREDDO: In a mixing bowl, combine sweetened condensed milk, lemon juice, lemon zest and whipping cream. Beat on high speed until thickened and soft peaks form. Fold in raspberries. Spoon mixture into a plastic wrapped 9x5-inch loaf pan. Freeze for 4-5 hours, or until firm. Cover if freezing overnight.
SAUCE: In a saucepan, combine raspberries, raspberry jam, orange juice and sugar. Bring mixture to a gentle boil, cooking until raspberries are very soft, about 8 minutes. In a small bowl, combine cornstarch, with water and mix thoroughly. Add to raspberry mixture, stirring constantly, until mixture thickens slightly. Remove from heat . Strain through a fine sieve to remove seeds. Cover and refrigerate.
To serve, unmold semifreddo from pan. Cut into slices and spoon over melba sauce.
Semifreddo can be wrapped well and frozen for up to five days. Melba Sauce can be covered and refrigerated two days ahead or frozen for two weeks.
TIPS: Spray loaf pan before fitting with plastic wrap. This helps wrap stay in place.