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Heat the oven to 180C/160C fan/gas 4 and line a brownie or square baking tin (about 20 x 20cm) with baking parchment. Set side. Drizzle enough oil into a large frying pan or saucepan (because a saucepan is high-sided, it's often easier to contain the herbs) set over a low-medium heat to coat the base. Tip in all the herbs and spring onions and cook for 10-15 mins until tender but not browned – when ready, the herbs will be dark green, like cooked spinach. Remove from the heat, season well with salt and pepper and stir in the barberries, if using. Leave to cool.
Crack the eggs into a large bowl and gently whisk along with the yogurt, baking powder and turmeric until the yogurt has dissolved into the mixture. Be careful not to overbeat the eggs, as this will make the batter stiff. Stir in the cooled herb mixture until the egg mixture turns completely green. Pour the mixture into the prepared tin and smooth the surface using a fork. Bake for 35-40 mins, or until a cutlery knife inserted into the middle comes out clean – there should be no remaining raw egg mixture. Serve warm or at room temperature, cut into small squares or triangles.