Fill a large pan with cold water, add a good pinch of sea salt and put it on a high heat. Peel 1 large swede (roughly 1kg), and cut it into 2cm chunks. Once the water is boiling, add the swede and cook for 15 minutes. Meanwhile, peel 1kg of potatoes and cut them into 2.5cm chunks. When the 15 minutes is up, add the potatoes and cook for a further 10 to 15 minutes, or until everything is soft and cooked through. Drain, leave to steam dry for a minute, then mash with a good pinch of salt and white pepper and a knob of butter.