Preheat the oven to 200°C (400°F/Gas 6). Peel the potatoes and cut into chunks, not too big. Keep in a bowl of cold water until you are ready to cook them.
Pour half the olive oil onto the bottom of a non-stick baking dish, about 22 x 30 cm (8½ x 12 inches). Pour the potatoes into a colander and immediately scatter the breadcrumbs over so they’ll stick to the clinging water. Turn through once with your hands, then tip the potatoes and crumbs into the baking dish. Scatter with the sage leaves (torn in half if very big), and salt and pepper. Add the rest of the olive oil and toss through quickly but thoroughly, making sure each potato is well coated.
Roast the potatoes for about 50 minutes, turning only after the bottoms are golden, about 35 minutes. Turn again until crusty and golden all over. Taste to check they have enough salt, then serve at once.