Mashed Sweet Potatoes with Caramelized Onions, Brie and Sage

medium to medium-large (orange flesh) sweet potatoes, peeled

medium onion, peeled and sliced thinly

olive oil

granulated white sugar

salted butter

brie pieces (see tips below)

to 1/2 cup nonfat milk

julienned fresh sage (see tips below)

salt and freshly ground pepper