Preheat the oven to 180 C.
Cut the leeks into roughly 1cm thick coins. In a large frying pan, heat the oil over a medium heat. Once the oil is hot, add the leeks and a generous pinch of salt to the pan then turn the heat down to medium-low. Let the leeks soften and try to avoid letting them colour, stirring often for 15-20 minutes. Add 60ml of water and stir until evaporated. Remove from the heat and set aside.
Cut the bread into roughly 3cm chunks. Layer the bread cubes with the watercress and cooked leeks into a medium roasting tin or casserole dish. Sprinkle over the grated cheese. Whisk together the eggs, milk, stock and a few grinds of black pepper. Season with a generous pinch of salt if you’re using low/no-salt stock. Pour this mixture evenly over the bread in the roasting tin. Let it sit for a few minutes then bake in the preheated oven for 35-45 minutes until dark-golden.
Remove from the oven and serve hot.