Preheat the oven to 175°C/350°F/gas 4. Prepare a tin measuring 20–22cm in diameter with baking paper at the bottom and around the sides.
Start with the crumble by mixing all the dry ingredients in a bowl and dicing the margarine. Rub the ingredients together to form a crumbly dough. Peel and cut the bananas into 5–7mm-thick rounds, or peel and core the apples and cut into thin slices, then dip in the cinnamon mixed with the sugar.
Now prepare the batter. Mix the flour, sugar, salt and baking powder in a bowl. Add the oil, milk and aquafaba to the bowl and mix with a spatula until smooth without overworking. Swipe the spatula around the sides of the bowl to incorporate any leftover flour.
Pour the batter into the tin and even it out. Cover the cake with the crumble mixture in an even layer and bake in the centre of the oven at 175°C/350°F/gas 4 for 30–35 minutes. Test with a skewer until it comes out clean from the middle of the cake.