Preheat the oven to 325°F and grease a 9 x 5” loaf pan with butter. Set aside.
In a pan over medium-low heat, toast the walnuts until warm to the touch and fragrant. Remove from the pan and allow to cool for five minutes. Give them a rough chop and set aside.
In a large bowl, sift together the all-purpose flour, rye flour, baking powder and soda, cinnamon, nutmeg, and kosher salt. Add the walnuts and stir until the walnuts are evenly distributed throughout the flour mixture. Set aside.
In a separate bowl, whisk the sugars and eggs together until well combined.
Add the oil and whisk until the oil is well incorporated.
Add the mashed bananas, yogurt, and vanilla to the egg mixture. Whisk until all of the ingredients have been thoroughly combined.
Add the dry ingredients to the wet ingredients and gently stir until just combined. Do not over mix the batter, as agitating the gluten in the all-purpose flour will make the bread chewy.
Evenly pour the batter into the greased loaf pan and sprinkle the turbinado sugar evenly over the top.
Bake for 1 hour and 10 minutes or until a toothpick inserted in the middle comes out clean. Allow the bread to cool in the pan for 15 minutes. Then, place it on a cooling rack and allow to rest for another 30 minutes before slicing into it. In the meanwhile, make the whipped maple brown butter.