Preheat your oven to 350°F (175°C) and grease a small oven-safe ramekin or loaf pan.
Place the chopped walnuts in a dry skillet over medium heat, tossing frequently for 2 to 3 minutes until fragrant and lightly toasted.
In a mixing bowl, mash the half banana with a fork until completely smooth and liquid.
Whisk the Greek yogurt, eggs, and vanilla extract into the mashed banana until the wet ingredients are fully combined.
Stir in the finely grated carrots until evenly distributed throughout the wet mixture.
Add the oat flour, ground cinnamon, baking powder, sea salt, and monk fruit sweetener, folding gently with a spatula until just combined.
Pour the batter into the prepared baking dish and sprinkle the toasted walnuts evenly over the top.
Bake for 25 to 30 minutes, or until the center is set and a toothpick comes out clean.
Allow the cake to cool for 5 minutes before serving warm.