Place the cubed squash and carrots in a large roasting tray, toss in half the oil and place in a pre-heated 200C (180C Fan) for 20 minutes (or until soft & lightly browned).
Using a large saucepan, gently fry the onions and garlic in the remaining oil for 5 minutes until the onions are translucent.
Peel and chop the sweet potatoes, add to the pan cover with water, season and simmer for 10-15 minutes.
Remove the roasting tray and add the contents to the pan (transfer some of the water from the pan to the roasting tray and heat for a few minutes to remove the oils etc from the tray and return to the pan).
Return to the pan, add the butter to melt and adjust seasoning if required.