Note - volume/weight of herbs is the chopped leaves, with the stems the weight is at least double so bear that in mind when buying herbs. The chopping also takes a bit of time, reflected in the prep time. While you are chopping the herbs, soak the barberries in water, if using.

Once all the herbs are chopped, whisk together the eggs, baking powder, salt and pepper until well mixed. Stir in the chopped herbs and drained barberries.

Warm the oil in an 8in/20cm skillet/frying pan. It should be warm, but you want the heat to be low. Pour the egg-herb mixture and flatten the top. Cover the pan and cook for a few minutes until you can see it has cooked at the sides and is set most of the way through. Loosen from the pan and cover the pan with a plate. Invert the pan so the frittata goes onto the plate then slide it off the plate, with what was the top now on the bottom. Cook another couple minutes until cooked through. Remove from the heat, cut and serve.