Preheat the oven to 200C/180C Fan/Gas 5. Line a baking tray with baking parchment.
Sift the flour into a mixing bowl and season with salt and pepper. Crack in the egg. Gradually add 300ml/10½fl oz water in a thin stream, beating continuously until the mixture forms a thick, smooth batter. Set aside for 1 hour to rest.
Meanwhile, for the filling, heat the butter in a frying pan over a medium heat. Add the onions and fry for 4-5 minutes, or until softened but not coloured.
Add the apples and continue to cook, stirring regularly, for 10-15 minutes, or until the apples have softened and started to brown. Season with salt, pepper and lemon juice. Set aside (keep warm).
When the batter has rested, stir in the melted butter until well combined.
Heat the ghee and a dash of olive oil in a ceramic or heavy-based frying pan over a medium-high heat. When the fat is smoking, add a tablespoonful of batter to the pan and swirl until the batter covers the base of the pan.
Fry the galette for 2-3 minutes, or until golden-brown on its underside and easy to loosen from the bottom of the pan. Flip over and fry for a minute on the other side, or until cooked through. Remove from the pan and keep warm in the oven. Repeat the process with the remaining batter.
Put a handful of rocket leaves in the centre of each galette. Divide the apple mixture equally among the galettes. Crumble over the feta.
Fold the edges of each galette over to create a square shape. Transfer them to the prepared baking tray and bake in the oven for 6-7 minutes, or until crisp. Serve with a simple green salad dressed with olive oil and a touch of apple cider vinegar.