Vegetarian Moussaka

• 2 large eggplants, cut into 5mm slices
• 1 brown onion, finely chopped
• 2 cloves garlic, crushed
• 400 g mushrooms, chopped
• 400 g tin brown lentils
• 700 g tomato passata
• 2 tsp oregano, dried
• 0.5 tsp cinnamon
• 50 g butter
• 50 g plain flour
• 500 ml milk
• 0.5 cup cheddar cheese, grated
• 1 egg