To make the caramel sauce: Combine all of the sauce ingredients in a heavy-bottomed saucepan set over low heat. Stir at regular intervals until the caramel has melted and the mixture is smooth.
Bring the sauce just to a boil, then remove from the heat.
To make the apple bowls: cut off the top portion of the apples, and hollow out the cores and surrounding flesh with a melon baller, spoon, or whatever other tool you have handy.
Brush the lemon juice all over the inside of each apple.
Fill each apple with 1/2 cup of vanilla ice cream, drizzle with caramel sauce, and top with toasted nuts, if desired.