Mix the vinegar with the honey and stir in the red cabbage. Leave to stand for at least an hour. This will give the red cabbage a nice bright pink colour.
Blend the feta with the Greek yoghurt in a food processor until smooth (or mash it together until slightly chunkier).
Coarsely chop the baby spinach. Spread one slice of bread with the feta mixture and top with the spinach. Place a second slice of bread on top and spread with hummus. Spread the red cabbage on top.
Cover with the last slice of bread and cut the sandwich in half.
