Grease the inside of your slow cooker with butter or cooking spray. Line with parchment paper or a slow cooker liner for easy removal. The batter will be layered in here, so ensure it's well-prepared.

In a medium bowl, whisk together the flour, baking soda, salt, cinnamon, and nutmeg. Set aside.

In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy, about 2 minutes.

Beat in the eggs one at a time, followed by the vanilla extract. Scrape down the sides of the bowl.

Fold in the mashed bananas and Greek yogurt. Mix until just combined.

Add the flour mixture to the wet ingredients and mix on low speed until just combined.

In a small bowl, beat together the cream cheese, sugar, egg yolk, and vanilla until smooth.

In another small bowl, mix flour, brown sugar, cinnamon with cold butter until crumble forms. Stir in walnuts if using.

Spoon half the batter into the slow cooker.

Pour the cream cheese mixture over it, keeping it centered.

Top with remaining batter and sprinkle streusel.

Swirl gently with a knife.