Grease a 9x5 inch loaf pan or line your slow cooker insert with parchment paper and grease it lightly to prevent sticking. Preheat the slow cooker to low.

In a large bowl, mash the ripe bananas until smooth with a few lumps. Whisk in the melted butter until combined.

Add the granulated sugar, brown sugar, eggs, Greek yogurt (or sour cream), and vanilla extract to the banana mixture. Whisk until uniform and silky smooth.

In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon to ensure even distribution.

Gently fold the dry ingredients into the wet ingredients using a rubber spatula, just until no streaks of flour remain. Avoid overmixing to keep the bread tender. Fold in walnuts if using.

Pour the batter into the prepared loaf pan or slow cooker insert. Place the pan into the slow cooker. Cover and cook on low for 2.5 to 3.5 hours, or until a toothpick inserted into the center comes out clean with a few moist crumbs.

Carefully remove the pan from the slow cooker (use tongs if hot). Let the bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely for the best texture.