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1
Put the strawberries, the 6 tablespoons sugar, the lemon juice, and the pinch of salt in a medium, heavy saucepan. Cook over medium heat, stirring frequently, until the strawberries reach a jam like consistency, 20 to 25 minutes. Let cool to room temperature and refrigerate for 1 hour until cold.
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2
Put the cream, milk, the remaining 1/4 cup sugar, and the 1/8 teaspoon salt in a medium, heavy saucepan. Cook over medium heat, stirring occasionally, until the liquid is almost simmering. Cool over an ice bath to room temperature. Refrigerate for 4 hours or up to overnight. Stir in the cooked strawberries. Churn in an ice cream machine according to the manufacturer's instructions. Freeze until scoopable, about 4 hours, depending on your freezer.
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3
Puree the raspberries in a food processor. Strain through a fine-mesh sieve, discarding the seeds. There should be about 2 cups puree. In a large pitcher, mix together the raspberry puree, sugar, lemon juice, and salt. Add the seltzer and stir until combined. Place several small scoops of strawberry ice cream and some blueberries and olallieberries in each of 6 glasses. Fill with the raspberry soda. Serve immediately, with long spoons and straws.
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4
The raspberry puree can be prepared and sweetened up to 12 hours before serving. Keep refrigerated. Add the seltzer and ice cream just before serving.
