In the bowl of a food processor, combine butter, sugars, cinnamon, salt and vanilla.
Pulse until coarse crumbs form.
Add flour and rolled oats and process for 30 seconds more.
You want some whole oat kernels remaining.
Turn mixture out into a large mixing bowl and set aside.
Core apples all the way through, slicing off about 1/4" of the tops. Scoop out flesh with a large spoon.
Leave about 1/4" of flesh on the sides and bottom of apples.
If you do not have an apple corer, simply slice off apple tops as instructed above,
scoop out flesh, remove and discard seeds.
Take the flesh that has been scooped out of apples (plus apple tops)
and coarsely chop into 1/4" - 1/2" pieces.
Toss with 1tsp fresh lemon juice.
Place apples, cut side up, into a glass or ceramic baking dish.
They should be tightly packed and standing up straight.
Add chopped apple pieces (and cranberries, if using) to the cinnamon-oat mix.
Stir to combine.
Spoon mixture into hollowed apples, pressing down to pack tightly.
Take any remaining filling and sprinkle around apples.
Bake at 350 degrees for 30-35 minutes or until apples have softened,
flesh is wrinkled and topping is lightly browned.
Some of the apples may have begun to split apart as well, this is a good thing!
You can test for doneness by inserting a sharp knife into one of their sides.
If it goes in like butter, you're golden.