Preheat the oven to 200°c.
Add the butternut squash, sweet potato and garlic to a large roasting dish.
Spray thoroughly with low calorie cooking spray and season with the sage, mustard and salt and pepper to taste. Make sure the chunks are evenly coated.
Loosely cover with foil and place into the middle of the oven for 60 minutes, or until the vegetables are cooked through and soft enough to mash.
Remove the baking tray from the oven and mash thoroughly. It won’t be smooth like potato mash but you will need to remove all the large lumps.