Preparation Crush vitamin C tablet in a bowl with back of a spoon, then line a sieve with dampened flour sack cloth and set over bowl. Purée apples with 1/3 cup water in a food processor (do not use a blender) until almost smooth, then pour into cloth. Bunch up cloth around mixture and squeeze as much clear juice as possible through cloth (about 11/2 cups). Discard solids remaining in cloth. Rinse and wring out cloth, then return to sieve. Purée tarragon with remaining 1/3 cup water in processor until water is deep green, then add celery and purée until almost smooth. Pour into lined sieve and squeeze out juices (about 1 1/2 cups) into bowl with apple juice. Stir in 1/2 teaspoon salt and 1/4 teaspoon pepper. Freeze mixture in an 8-inch square baking dish, scraping and stirring with a fork every 30 minutes, until frozen, at least 3 hours. (It will be too hard to scrape once fully frozen.) Cooks’ note:Granita can be made 2 days ahead (cover once frozen). Let stand at room temperature about 10 minutes and re-scrape before serving.
Preparation Crush vitamin C tablet in a bowl with back of a spoon, then line a sieve with dampened flour sack cloth and set over bowl. Purée apples with 1/3 cup water in a food processor (do not use a blender) until almost smooth, then pour into cloth. Bunch up cloth around mixture and squeeze as much clear juice as possible through cloth (about 11/2 cups). Discard solids remaining in cloth. Rinse and wring out cloth, then return to sieve. Purée tarragon with remaining 1/3 cup water in processor until water is deep green, then add celery and purée until almost smooth. Pour into lined sieve and squeeze out juices (about 1 1/2 cups) into bowl with apple juice. Stir in 1/2 teaspoon salt and 1/4 teaspoon pepper. Freeze mixture in an 8-inch square baking dish, scraping and stirring with a fork every 30 minutes, until frozen, at least 3 hours. (It will be too hard to scrape once fully frozen.) Cooks’ note:Granita can be made 2 days ahead (cover once frozen). Let stand at room temperature about 10 minutes and re-scrape before serving.