Preheat oven to 170 degrees celsius (160 degrees if fan forced). Grease and line a 17X28cm slice tray with baking paper, allowing the edges to overhang.
Melt the butter and vanilla in the Thermomix bowl for 3 minutes, Speed 2, 50 degrees.
Add the plain flour, rolled oats, salt, bi-carb soda, brown sugar and raw sugar. Mix for 20 seconds on Reverse, Speed 2. Scrape down the sides. Mix for a further 20 seconds on Reverse, Speed 2.
Spoon half of the crumble mixture into the bottom of the prepared tin and press very firmly into an even layer. Bake in preheated oven for 15 minutes.
Remove from oven and set aside (leave the oven turned on).
To make the apple filling, place the sliced apples into the Thermomix bowl. Add the plain flour, raw sugar, cinnamon and nutmeg and mix on Reverse, Speed 2 until coated (use the spatula to help mix).
Spread the apple slices in an even layer over the top of the cooked base.
Sprinkle the remaining crumble mixture over the top of the apple and press down lightly.
Bake in oven for 25 minutes or until lightly golden.
Remove from oven and allow to cool.
Cover and refrigerate for at least 2 hours (preferably overnight) then take it out and cut into squares.
Store in airtight container in refrigerator for up to 4 days or freeze for up to 3 months.