Preheat the oven to 180°C fan/200°C/400°F/Gas mark 6 and line a 20cm loose-bottomed cake tin with parchment, then brush it with olive oil.
Combine the flour, both sugars, baking powder and salt in a mixing bowl. Break the eggs into a separate bowl, then add the oat milk and olive oil. Using a balloon whisk, beat well. Tip the dry ingredients into the bowl with the wet, then whisk everything together until smooth.
Pour the batter into the prepared tin and scatter the berries over the surface. Bake in the middle of the hot oven for 40 minutes or until a toothpick inserted into the centre comes out clean. Leave to cool in the tin for 10 minutes.
Place the membrillo in a small pan with 1 teaspoon of water. Gently melt the membrillo over a low heat. Using a pastry brush, spread the membrillo glaze over the top. Leave the glaze to cool before removing the cake from the tin. Serve in slices either slightly warm or cool. It’s great with crème fraîche.