Step 1 Toss the strawberries, 1/2 cup sugar, orange juice and balsamic vinegar in a large bowl until evenly coated. Taste and add more sugar if needed; the mixture should be quite sweet, and freezing will dull the flavors. Arrange on a rimmed baking sheet in a single layer, and freeze until solid, about 2 hours.
Step 2 Scrape the frozen berries into a food processor, add the mascarpone and pulse, scraping down the sides frequently, until creamy, 3 to 4 minutes. At first there will be large chunks of ice, but eventually it will turn into a smooth mixture.
Step 3Serve immediately or spoon into a covered dish and freeze until ready to serve. If the mixture refreezes solid, thaw slightly, cut into small chunks, return to the food processor and pulse until creamy.