Boil the potatoes in skins for 20-25 minutes, until just tender. Leave to cool, peel, then coarsely grate into a bowl.
Heat 1 tbsp of olive oil in a medium non-stick frying pan, add the onion and lardons and fry for 7-8 minutes, stirring occasionally, until the onion is softened and lightly tinged with brown. Stir in the sage leaves and remove from the heat.
Stir the onion, lardons and any juices left in the pan into the grated potato. Season with salt and pepper and mix well.
Heat 1 tbsp of oil in the frying pan, add half the potato mixture and spread over the pan. Crumble the goat’s cheese into small pieces, then sprinkle evenly over the potatoes. Spread the remaining potato over the top to seal in the cheese and press down lightly with the back of a wooden spoon.
Cook the rösti over a moderate heat for 8 minutes until the underside is nicely browned, then put a large plate on top and invert the rösti onto it. Add another tablespoon of oil to the pan if you need to, then slide the rösti back into the pan and cook the other side for 8 minutes. Serve the rösti straight from the pan, cut into wedges, with green vegetables or a big green leaf salad.