Peel and dice the onions. Heat the olive oil in a pan and saute the onions until softened.
Wash and dice the courgettes . Add to the onions, cook for 2-3 minutes, then add the tomato paste.
Stir well, simmer briefly and pour in the chopped tomatoes. Simmer for 10 minutes, season with salt and pepper and set aside.
Mix the flour with the baking powder and 2 pinches of salt.
Add the remaining ingredients for the batter and stir until smooth.
Leave to rest for 15 minutes, then stir once more well.
Dice the feta.
Heat a deep fat fryer or deep pan half-filled with oil to 170°C. Turn the feta cubes in the flour, dip individually in the batter to coat and fry in batches until golden brown.
Drain on kitchen paper.
Heat the sauce again and stir in the basil. Spoon onto plates and scatter the feta cubes over. Garnish with lettuce and parsley.