Lightly grease a baking sheet with the butter. Chop the cabbage and feta cheese then mix together the mashed potato, butternut squash, cabbage and feta cheese and season with a pinch of salt and a couple of twists of freshly-ground black pepper. Use your hands to form the mixture into 8 patties, dusting them with a little flour. Place them on the baking sheet, then chill for at least 15 minutes.
Meanwhile, preheat the oven to moderate (Mk 4 - 350ºF - 180ºC). Bake the patties for 20-25 minutes, until piping hot. Serve 2 patties per person.