Step 1Peel the 3 pounds of mangoes and slice the fruit from their pits. Scrape as much pulp off the fibrous pits as you can. In a food processor, puree the fruit and pulp, about 2 minutes. Measure 3 1/2 cups from the puree and set it aside in a mixing bowl.
Step 2In a small saucepan, gently simmer the sugar with one-half cup water for 5 minutes over medium-low heat to allow the sugar to dissolve and the syrup to reduce slightly. Cool the syrup, then add it to the mango puree with the lime juice.
Step 3Chill thoroughly; then freeze according to the directions for your ice cream maker.
Step 4Scoop about one-half cup of the sherbet into each of 8 small bowls. Scatter 1 tablespoon of the diced mango over each serving and scatter the julienned shiso leaves evenly over the mango in each. Drizzle about one-half tablespoon of the Sauternes over each serving and serve immediately.