Preheat the oven to 200C (400F), Gas mark 6. Heat the oil in a frying pan and sauté the onions, garlic and herbs for 10 minutes until soft and lightly golden. Season to taste. Layer the potatoes in a large baking dish with the onions and celeriac. Season each layer as you go and finish with an attractive overlapping layer of the potatoes. Pour over the stock and top with knobs of butter.
Rub some foil with olive oil and place it, oil side down over the dish and seal tight. Bake for 45 minutes, then remove the foil, press the potatoes down with a fish slice. Scatter over the breadcrumbs and bake for another 15-20 minutes until golden.