Preheat the oven to 350°F (about 175–180°C) and grease or line an 8‑inch round cake tin.
In a large bowl, mix the rolled oats, grated carrots, grated apple, chopped walnuts, raisins, ground cinnamon and a pinch of salt.
Add the eggs, Greek yogurt or sour cream, honey, melted oil and lemon zest, and stir until everything is well combined.
In a small cup, mix the baking soda with lemon juice; once it foams, add it to the batter and stir gently.
Pour the batter into the prepared tin and smooth the top.
For the crunchy topping, mix the oats, cinnamon, melted oil and a little sweetener, then sprinkle over the batter.
Bake for 50–55 minutes, until the cake is set, lightly golden and a skewer inserted into the centre comes out clean.
Cool in the tin for 10–15 minutes, then transfer to a rack and let the cake cool completely.
For the frosting, mix the cream cheese or ricotta with Greek yogurt, vanilla and a splash of lemon juice until smooth.
Spread the frosting over the cooled cake and finish with chopped walnuts on top.