Preheat oven to 350 degrees F / 176 degrees C. Grease a 9 inch / 23 cm springform pan and line the bottom with parchment paper. If you don't have a springform pan, you can use a 9x13 / 23x33 cm pan.
In mixing bowl, stir together flours, oats, baking powder, baking soda, salt and nutmeg.
In separate bowl, beat together vegetable oil, brown sugar, bananas, eggs, vanilla and buttermilk until thoroughly mixed. Add flour mixture and mix gently to combine until there are no streaks of flour visible. Do not overmix.
Spoon mixture into prepared pan. Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean. (It will not be dry, but there should be no batter clinging to the toothpick.)
Remove the cake from the oven. If you are using a springform pan, let the cake sit for 15 minutes, then run a knife around the side of the pan before releasing the sides. Let cake cool completely and then spoon the topping over. If you are using an rectangular baking pan, you can let the cake cool in the pan, or turn it out onto a cooling rack and spoon topping over when cooled.
While the cake is cooling, make the topping. Combine butter, brown sugar and cream in a small saucepan. Stir until butter has melted, then bring to a boil over medium heat. Let the mixture boil for one minute, then remove from heat and stir in coconut and pecans. When cake has cooled, spoon the topping over the cake.
For a vegan version, substitute 1/4 cup / 60 ml aqua faba for the eggs, and 1 cup / 250 ml non-dairy milk + 1 tablespoon / 15 ml apple cider vinegar for the buttermilk.
For the topping, use vegan butter and substitute coconut milk for the cream.