Vegetarian pastitsio

Name: Vegetarian pastitsio
Ingredients: 2 large onions (330g), halved and sliced; 1 tbsp olive oil; 2 tsp vegetable bouillon powder; 2 bay leaves; 2 tbsp balsamic vinegar; 2 garlic cloves, finely grated; 1 tsp dried oregano; 1 tsp ground cinnamon; 250g penne pasta; 400g can chopped tomatoes; 4 tbsp tomato purée; 2 x 400g cans green lentils; salad of tomatoes, onion and dill tossed with lemon and olives, to serve; 500ml milk or plant-based milk; 50g wholemeal flour; 1 egg, separated; 40g vegetarian feta, crumbled; ½ tsp oregano