Preheat the oven to gas 6, 200ºC, fan 180ºC.
In a very large frying pan (or two regular ones), melt the butter. Add the onions, leeks, celery, thyme, sage and salt. Cook on medium-low heat for 15 minutes or until all the vegetables are softened.
Put the bread cubes in a large bowl and pour over the veg. Mix everything well to combine and then pour in the eggs and stock. Stir, then season to taste.
Put the mixture in a baking dish and cover with foil. Bake for 20 minutes, then remove the foil and cook for a further 10 minutes, or until golden.
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