Preheat the Oven and Prepare the Pan: Preheat your oven to 180°C (350°F). Grease or line a baking pan with parchment paper for easy removal after baking.
Mix Wet Ingredients: In a large bowl, whisk together the milk, eggs, vanilla, and sweetener. Stir until smooth and combined.
Add the Fruits and Yogurt: Stir in the mashed banana, diced apple, and yogurt to the wet mixture. Mix well until everything is evenly distributed.
Combine Dry Ingredients: In a separate bowl, mix the oatmeal, cinnamon, and baking powder. Slowly fold the dry ingredients into the wet mixture until well combined. Stir in the walnuts.
Bake the Cake: Pour the batter into the prepared pan. Bake in the preheated oven for 25–30 minutes, or until the top is golden brown and a toothpick inserted into the center comes out clean.
Cool and Serve: Allow the cake to cool for a few minutes before slicing. Serve warm for a comforting breakfast or enjoy at room temperature.