Heat half the oil in a frying pan and gently fry the onions for about 10 mins until pale and soft, then leave to cool a little.

Finely chop the spinach in a food processor and tip into a bowl. Add the cooled onion, breadcrumbs (which you can make in your blender), nutmeg, cheddar and Parmesan, and mash together. Add the beaten egg, a little at a time (you may not need all of it), until the mixture holds together. Divide into even patties (see tip below) and shape into fat burgers.

Put the flour in a shallow bowl, season well and dip the burger patties into the flour to coat. Store in a plastic container between layers of baking parchment. Either chill until ready to cook, or freeze. If serving with sweet potato fries, put them in the oven now and cook according to the packet instructions.

Heat the remaining oil in the frying pan and fry for about 5 mins each side until browned all over. Serve in the crusty rolls or burger buns, with a couple of slices of tomato, ketchup and the sweet potato fries on the side, if you like.