Lightly grease the bottom and sides of your slow cooker insert with vegetable oil or butter. Sprinkle a little flour or line with parchment paper for easy removal. In a dry skillet, toast the walnuts over medium heat for about 5 minutes until fragrant. Set aside to cool, reserving 1/4 cup for topping.

In a large mixing bowl, mash the bananas until largely smooth, leaving some small chunks for texture. Whisk in the vegetable oil, eggs, buttermilk, and vanilla until well combined and slightly lightened.

In a separate medium bowl, whisk together the flour, granulated sugar, baking soda, salt, and cinnamon.

Pour the dry ingredients into the wet ingredients. Mix gently with a spatula until just combined. A few streaks of flour are fine; do not overmix.

Fold in 3/4 cup of toasted walnuts. Sprinkle the reserved walnuts over the top.

Pour the batter into the prepared slow cooker. Cover and cook on low for 3 hours, or until a wooden skewer inserted in the center comes out clean. If needed, check at 2.5 hours. Do not lift the lid frequently to maintain even cooking.

Turn off the slow cooker and let the bread cool inside for 15 minutes. Carefully remove using parchment if lined, or invert onto a wire rack to cool completely before slicing.