Preheat oven to 200°C. Set aside medium baking dish (measuring about 20cm x 20cm). Scoop out feijoa flesh and roughly chop. Peel apples, remove core and dice 2cm.
Add feijoas, apples, sugar and first measure of water to a medium pot and bring to the boil. Once boiling, reduce heat to medium and simmer for about 5 minutes, until tender. Transfer mix to reserved dish and reserve pot.
Add vanilla, butter, second measure of water and golden syrup to reserved pot and cook on a low heat for about 2 minutes, until butter is melted and combined.
Add all remaining crumble ingredients to pot and mix to combine.
Sprinkle crumble topping over filling and bake for 15-20 minutes, until crumble topping is golden.
To serve, spoon crumble into bowls and serve with yoghurt, whipped cream or ice cream (if using).