To make sauce, pour apple juice into a small pot. Whisk in the cornstarch until dissolved; mix in the blueberries, vanilla and cinnamon. Set over medium-high heat and bring to a simmer. Simmer 1 to 2 minutes, or until the sauce is lightly thickened.
Cool sauce to room temperature, and then refrigerate until needed.
Set 2 to 3 scoops of ice cream in each of 4 chilled decorative glasses. Spoon some blueberry sauce over the ice cream in each glass. (Any leftover sauce can be saved for another time.) Sprinkle the sundaes with almonds. Pipe or dollop some whipped cream on each sundae, if using. Garnish each sundae with a mint sprig and serve.
Note: To toast the almonds, place them in a single layer in a small baking pan. Bake in a 300°F oven for 10 to 12 minutes, or until lightly toasted.
Recipe Options: Make blackberry sauce for the sundaes by replacing the blueberries with fresh blackberries. Instead of vanilla ice cream, try another flavour in these sundaes, such as chocolate or, for a two-fruit taste, strawberry. Instead of almonds, sprinkle sundaes with pecan pieces.