1 cup fresh grapefruit juice (from 1 or 2 large grapefruit, preferably pink)
3 tablespoons Champagne vinegar
1/2 tablespoon sugar dissolved in 1/2 tbsp. hot water
1/2 tablespoon vodka
1/2 teaspoon finely chopped tarragon
1/2 teaspoon chopped flat-leaf parsley
1/2 teaspoon freshly ground black pepper
Rock salt or crushed ice (see Notes)
3 dozen chilled small to medium oysters on the half-shell
How to Make It
Combine all ingredients except rock salt and oysters, pour into a loaf pan (to a depth of about 1 in.), and freeze 1 hour. Rake and crush mixture with the tines of a fork and freeze for another hour. Rake and crush mixture again and freeze 1 hour more.
Spread rock salt on a platter. Rake and crush granita a final time into fine crystals. Set oysters in place and top each with a spoonful of granita.