Cut off the tops of 4 apples. Using a knife, core the apples, creating a bowl (leaving the bottoms intact). Using a grapefruit spoon, dig out the inside.
Place hollowed apples onto a baking sheet or pan. Cut apple insides and tops into small chunks.
In a bowl, combine granulated sugar, cornstarch, 1/2 teaspoon cinnamon, 1/4 teaspoon nutmeg, and 1/8 teaspoon salt. Mix until combined.
Mix apple chunks, lemon juice, and vanilla into sugar mixture until coated. Spoon filling into the hollowed-out apples and pour 1 tablespoon caramel sauce into each apple.
In a bowl, whisk together flour, oats, pecans, brown sugar, 1/2 teaspoon cinnamon, 1/4 teaspoon nutmeg, and 1/8 teaspoon salt. Cut in the cold butter until small clumps form.
Sprinkle streusel over top of stuffed apples, allowing excess streusel to fall onto the bottom of the baking sheet.
Place apples in a 375 F oven and bake for 25 to 30 minutes or until streusel is browned and filling is bubbly. Remove from oven and allow to cool 10 minutes before serving. Top with vanilla ice cream and more caramel sauce, if desired.