1. Place an 8-inch square baking pan in the freezer. Puree strawberries with sugar and salt in a food processor. Add 1 1/2 cups water and lemon juice and pulse to combine. Pour through a large fine-mesh sieve set over a large glass measure or bowl, pressing hard on solids to extract as much liquid as possible. Pour mixture into cold baking pan.
2. Cover tightly with aluminum foil and freeze for 1 hour, or until ice crystals form around edges. Stir mixture well with a fork to incorporate ice and break up any chunks. Continue to freeze, scraping ice at edges into center and breaking up pieces with a fork every 30 minutes, for 2 1/2 to 3 hours, until granita is granular but still slightly slushy.
3. Use a fork to scrape across granita surface, transferring ice shards to glasses or bowls, and serve immediately, garnished with whipped cream, if desired.