Place the strawberries in the bowl of a food processor fitted with the steel blade and puree until completely smooth. Pour the liquid through a fine-mesh sieve and press to make a seedless puree. Discard the seeds. Add the sugar syrup and Grand Marnier to the puree and refrigerate until very cold.
When ready to serve, rub the rim of each Champagne glass with the orange zest. Pour the cold Prosecco into Champagne glasses until they're each three-quarters full. Carefully stir 2 tablespoons of the puree into each glass and serve very cold.
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