Instructions:

1. In a large bowl, mix the vinegar with the milk and let it sit for 10 minutes to sour.
   
2. Meanwhile, wash and dry the orange. Using a fine grater, zest the orange, being careful to avoid the bitter white pith.
   
3. Peel the orange and slice it crosswise into rounds, about 5-7 mm (1/4 inch) thick.
   
4. Line the bottom and sides of a skillet with parchment paper. Sprinkle the bottom with a pinch of sugar and arrange the orange slices in a single layer, with the wider sides facing down.
   
5. Once the milk has soured, add the flour, orange zest, remaining sugar, salt, baking soda, egg, and 1 tbsp of oil. Mix well until the batter is smooth.
   
6. Pour the batter over the orange slices, spreading it evenly.
   
7. Cover the skillet with a lid and cook over low heat until the surface is set and no longer wet (about 10–15 minutes).
   
8. Carefully transfer the cake onto a plate, holding the parchment paper. Lightly grease the skillet with sunflower oil, then return the cake to the pan, orange slices facing up. Cook for another 3 minutes over low heat.