3 potatoes, scrubbed but unpeeled, about 625 g/1 1/4 lb total weight
1/2 onion, very thinly sliced
4 tablespoons vegetable oil
50 g (2 oz) butter
1 garlic clove, chopped
250 g (8 oz) button mushrooms, thinly sliced
2 tablespoons finely chopped parsley (optional)
salt and pepper
1 large bunch of watercress, to serve
Cook the potatoes whole in a large saucepan of lightly salted boiling water for 8–10 minutes. Drain and set aside to cool slightly. Wearing rubber gloves to protect your hands from the heat, coarsely grate the potatoes and mix in a bowl with the sliced onion, 2 tablespoons of the oil and plenty of salt and pepper.
Heat the remaining oil in a large nonstick frying pan and add the rosti mixture, pushing down to flatten it so that it covers the base of the pan. Cook for 7–8 minutes, then slide on to an oiled plate or board. Flip the rosti back into the pan to cook the other side for 7–8 minutes until crisp and golden.
Meanwhile, melt the butter in a frying pan and cook the garlic and mushrooms gently for 6–7 minutes, until softened and golden. Season to taste with salt and pepper, then stir in the chopped parsley, if using.
Cut the rosti into wedges, then arrange on serving plates, scatter over the watercress and spoon over the warm mushrooms with their juices. Serve immediately.