Bring two pans of water to the boil and add 1 tsp of salt to each pan.
Add the swede to one pan and cook for 20-25 minutes until the swede has softened and can be mashed.
After around 10 minutes, cook the potatoes in the second pot for 15-20 minutes until the potatoes soften enough to mash.
Drain each vegetable separately, then return to the dry but warm pan. Lightly mash with the back of a fork while folding through 30g of the butter through each vegetable. Season to taste and serve separately.
Alternatively, add the potatoes to the cooking swede water for 15 minutes, drain them together, and mash them with all the butter. Season to taste.