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Heat the oven to 190C/170C fan/gas 5. Put the onions and oil in a large saucepan with the bouillon, bay leaves, the vinegar, garlic, oregano and cinnamon. Add a mug of water (about 250ml), then cover and simmer for 10 mins until soft. Uncover and cook for 10 mins more, stirring frequently. The water should evaporate after this time and the onions should be soft.
Meanwhile, cook the pasta in a large pan of boiling water following pack instructions, or for 10 mins until al dente. Drain and set aside.
Stir the tomatoes, tomato purée and the lentils, along with their liquid, into the onions. Stir well, then cover and simmer for 5 mins. Remove the bay leaves.
To make the white sauce, pour the milk and 200ml water into a pan with the flour, whisk well to stop lumps forming and keep stirring until thickened, for about 8-10 mins.
Stir the egg white into the pasta and spoon over the base of a baking dish. Top with the lentil mixture. Stir the egg yolk into the white sauce, then spoon on top of the lentils, scatter over the feta and oregano, then bake in the oven for 40 mins until bubbling.
Serve half with the tomato and onion salad, and chill the rest for another day. Will keep chilled for up to two days. Reheat in the microwave on plates until piping hot.