Heat oven to 190C/170C fan/gas 5. Line a muffin tin with cases. Heat the oil in a pan and cook the onion until soft.
Quarter the beef tomato, scoop out the seeds and core, then discard and very finely chop the tomato and olives, if using. Crumble the feta into really small crumbs. Set everything aside.
Set a sieve over a large mixing bowl. Measure out the flours, polenta, bicarb and baking powder along with 1 tsp salt, decanting directly into the sieve as you go. Use the back of a spoon to press everything through the sieve – there will be some wholemeal flour left over in the sieve at the end – just tip this back into the mixing bowl.
Beat the egg in a separate bowl. Measure out the sunflower oil and milk and stir into the egg. Make a well in the flour mixture, tip in the egg mix and stir it together. Fold in the onion, tomato, feta, oregano and olives, if using. Mix well. Spoon the mixture between the muffin cases. Grate over a little cheese and bake for 25 mins. Eat warm or leave to cool and pack in a picnic.