Vegetarian eggplant moussaka gratin
Ingredients2 tbsp olive oil4 red onions, peeled, thinly sliced4 garlic cloves, crushed2 tbsp tomato paste400ml chicken or vegetable stock400g canned crushed tomatoes400ml red wine1 cinnamon quill3 tbsp finely chopped oregano, plus extra leaves to serve½ tsp dried oregano, plus a pinch2 medium eggplants (about 650g-750g), diced into 4cm cubes800g potato, sliced lengthways using a mandolin