Preheat the oven to 200C (400F) and grease a medium roasting dish (around 1.5 litre capacity) with a bit of extra olive oil.
Heat 1 tbsp of the olive oil in a large frying pan over a medium heat. Add the coins of sliced leek and season with a pinch of salt. Stir to coat with oil and turn the heat to low. Leave to cook for 5-10 minutes, stirring every now and then, until soft. Add the garlic and cook, stirring, for 1 more minute. Tip the cooked leeks and garlic into a large bowl.
Place the frying pan back on the heat and add the mushrooms to the pan with a pinch of salt. Allow to cook, stirring often, until most of the water has been drawn out and evaporated from the mushrooms. Once you see them start to brown, add the remaining tablespoon of olive oil, the white pepper and soy sauce and let cook until reduced and dry. Set aside to cool.
To the bowl of leeks add the vegetable broth, rosemary, thyme and bread cubes. Stir to allow the bread to soak up the liquid. Tip into the prepared roasting dish. Dot with spoonfuls of the soy sauce mushrooms. Drizzle with a bit of extra olive oil.
Bake for 25-35 minutes until the top is crisp and starting to brown. Serve hot.