Start by gently frying the chopped onions with a little olive oil over a low heat until golden brown (a good ten minutes or so). Add the wild mushrooms and simmer over a medium heat until all the water has evaporated.
Beat the eggs with the cream and add the Herbes de Provence, salt and pepper. Arrange the rashers of bacon lengthwise in a sponge-cake mould. Add a layer of sliced bread, followed by a layer of onions and mushrooms, and then by alternating layers of sliced bread / onions and mushrooms until you have filled the cake mould, finishing off with a layer of bacon. Add the cream and eggs and leave to soak in. Add some more rashers of bacon. Cover with tinfoil and cook over a main-marie for 90 minutes at 150 ºC.
For the leek sauce: wash and finely chop the leeks. Lightly fry in butter with a pinch of salt. Add the vegetable stock and leave to simmer and reduce. Then give it a whiz with the blender and strain to give a fine liquid.
Take the terrine out of the mould, cut into slices and serve with the hot sauce.