Pre-heat the oven to 160°C / 140°C / gas mark 2 and line and grease a 20cm deep round cake tin
First make the streusel topping by rubbing together the flours, sugar, cinnamon, butter and oats until the mixture is crumbly, then set aside.
In a large mixing bowl beat together the caster sugar, light brown sugar, olive oil, stem ginger syrup, eggs and vanilla until completely combined.
In a separate bowl sift together the flours, salt, baking powder, cinnamon, nutmeg and ground ginger.
Add the flour to the sugar and egg mixture and beat until well combined.
Stir in the green tomatoes and the stem ginger until evenly distributed then pour into the cake tin.
Sprinkle the streusel topping over the cake mixture.
Place the cake in the oven and bake for about 90-100 minutes until an inserted cocktail stick comes out clean. You might want to check the cake two thirds of the way through its cooking time and cover the top with foil if the streusel topping is getting too brown.
Remove the cake from the oven and leave for 5 minutes in the tin before turning out to finish cooling on a wire rack.