Peel the sweet potatoes, thickly slice, put into a 2 quart saucepan, and cover with cold, salted water.
Bring to a boil, reduce heat, and simmer covered until tender, about 20 minutes.
Drain and chop coarsely.
In a large frying pan, heat butter on medium.
Sauté the onion until translucent.
Add the tomatoes and cook for about 5 minutes longer.
Put the mixture into a blender or food processor with the sweet potatoes and 1 cup of the stock.
Reduce to a smooth purée.
Pour purée and rest of stock into saucepan.
Season with salt and pepper and reheat.
Garnish with chopped parsley.