Preheat the oven to 170C. Grease lightly a 20 x 30cm tin.
Place flours, oats, brown sugar, walnuts and melted butter in a kitchen processor and blitz to form a soft crumble dough.
Press half of the mixture into the base of the tin.
Combine the apple cubes, sugar, zest and feijoa in a bowl. Spread over the base.
Top with the remaining dough, pressing gently around with the back of a fork.
Bake in the oven for 45 minutes until golden. Remove and cool before cutting into pieces.
Store in an airtight container.