Peel the onions, cut them in half and thinly slice them. Put 1 tablespoon of oil in a frying pan and when hot, add the onions and cook them very gently for about 20 minutes until they are soft and beginning to colour (but not brown!). A pinch of salt will help the onions to soften without colouring.
Whilst the onions are cooking, peel the carrots, cut them in half and then thinly slice into half moons. Thinly slice the celery, wash and peel the outer leaves from the leek and then slice that very thinly too.
After 20 minutes, add the carrots, celery and leek to the pan and continue to cook gently for another 10 minutes, turning regularly. You are not looking to fully cook the carrots, celery and leeks, just to start the process. After 10 minutes, put all of the vegetables into a large casserole dish, add the tomato ketchup and give it a good stir
Preheat the oven to 180℃, Gas Mark 4
Put the seasoned flour onto a deep plate or bowl and then coat each slice of liver in the flour. In the frying pan you used for the onions and other vegetables, add the remaining two tablespoons of oil and fry each slice of liver for one minute each side, then add the liver to the casserole. Don't crowd the pan with the liver, cook it in several batches if necessary
Using a tablespoon or so of the stock, deglaze the frying pan to get all the crusty bits off the bottom and pour this over the casserole dish. Season the casserole with plenty of salt and pepper. Very gently turn the casserole to mix the liver and the vegetables together (you don't want to break up the liver). Pour the remaining stock over the casserole, add the worcestershire sauce, the oregano and the sage and give it a gentle stir. Put in the preheated oven for 45 minutes
Mix the cornflour with a little cold water and, after 45 minutes of cooking add this to the casserole and stir in gently to thicken the gravy slightly. Cook for another 15 minutes and it's done! Leave it overnight to mature and then heat it up and serve it with an accompaniment of your choice. Enjoy with a glass of red!