Vegetarian moussaka

Name: Vegetarian moussaka
Ingredients: 140g dried green lentils; 2 onions, halved and sliced; 2 garlic cloves, chopped; 2 bay leaves; 1 tsp dried oregano; ½ tsp cinnamon; ½tsp allspice; 400g can chopped tomatoes; 1 cube reduced-salt vegetable stock; 200g sweet potato, thinly sliced; 1 large aubergine, sliced and the biggest slices halved again; 300g full-fat plain yogurt; 60g plain flour; 2 egg yolks; 120g mature cheddar, finely grated; 2-3 tomatoes, cut into slices (optional)