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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, 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 eight (see tip below) and shape into fat burgers.
Put the flour in a shallow bowl, season well and dip the burgers into the flour to coat. Store in a plastic container between layers of baking parchment. Either chill until ready to cook, or freeze.
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, with a couple of slices of tomato, ketchup and sweet potato fries on the side, if you like.