Vegetarian eggplant moussaka gratin

Name: Vegetarian eggplant moussaka gratin
Ingredients: 2 tbsp olive oil; 4 red onions, peeled, thinly sliced; 4 garlic cloves, crushed; 2 tbsp tomato paste; 400ml chicken or vegetable stock; 400g canned crushed tomatoes; 400ml red wine; 1 cinnamon quill; 3 tbsp finely chopped oregano, plus extra leaves to serve; ½ tsp dried oregano, plus a pinch; 2 medium eggplants (about 650g-750g), diced into 4cm cubes; 800g potato, sliced lengthways using a mandolin (skin on); Cheese sauce; 50g butter; 70g plain flour; ½ tsp freshly ground nutmeg; 800ml milk; 80g parmesan cheese, grated; 80g kefalotyri cheese, grated (or use pecorino); 2 egg yolks; salt and pepper to season; ½ tsp freshly ground nutmeg, plus a pinch