Great use for butternut squash in season. I get mine in farmers market (around September, October).
about 2l pot
Dice your onions and put to fry on butter, low heat
Peel and dice your butternut and potato
Add all to pan and saute nicely
Add 2 cups of water and boil for about 10 minutes
Add milk, salt, pepper and blend it
Cook a bit more (not really necessary I think)