Remove the end of the squash with the stem and then divide the whole thing in two by cutting it near the bulbous base. Then halve both of those sections lengthwise.
Scoop out the seeds from the base and cut the squash into equal parts.
Place the pieces in a baking dish with a bit of water, about 1/8 inch. Cover the dish with tinfoil and roast at 425 degrees for about an hour, or until the flesh is tender.
Remove the baking dish from the stove and let the squash cool. Once cooled, scoop out the flesh into a large mixing bow.
Peel the russet potatoes and store them in cold water. This prevents them from browning. If you happen to like that rustic look (I do!) then leaves some of the skin on.
Cut the potatoes into equal-sized pieces and place in a large pot with just enough water to cover the potatoes. Salt the water and bring it to a boil.
Simmer the potatoes for about 20 minutes or until a fork inserted into the potato meets no resistance. If the potato holds to the fork, they need to cook longer.
Drain them immediately when done, reserving the water for soup stalk. You can also use it as a vegan option for mashing the potatoes.
Add the cooked butternut squash, one cooked russet potato and a tablespoon of brown sugar to a large mixing bowl.
Mash until desired consistency.