Bring cream, 1/2 cup sugar and lemon zest to a simmer in a large saucepan over medium-low heat. Meanwhile, in a small bowl, soften gelatin in 2 generous Tbs. cold water. Whisk softened gelatin and 2 Tbs. orange liqueur into cream mixture.
Pour cream mixture into 8 4-ounce ramekins or custard cups that have been lightly coated with vegetable cooking spray (blot excess oil with a paper towel). Set custard cups in a shallow pan, cover with plastic wrap and refrigerate until set, at least 4 hours, preferably overnight.
Partially thaw raspberries on countertop. Place in a food processor fitted with the metal blade and add remaining 6 Tbs. sugar and 2 Tbs. orange liqueur. Transfer sauce to a medium bowl (if you want a seedless sauce, strain first through a fine-mesh sieve). Stir in fresh raspberries, and let stand about 1 hour.
When ready to serve, run a thin-bladed knife around each dessert to loosen; turn onto a dessert plate to unmold. Spoon raspberry sauce around panna cotta and serve.
