Preheat oven to 375°F. Steam sweet potato until fork-tender, about 15 minutes. Mash until smooth. In a medium saucepan, heat oil over medium heat. Add onion and garlic, and cook, stirring frequently, until onion is softened and translucent, about four minutes. Add flour and cook, stirring constantly, for about one minute, until golden brown. Stir in mustard and sweet potato. Whisk in evaporated milk and stock until smooth. Bring to a gentle simmer and cook, stirring frequently, until thickened, about five minutes. Remove from heat and gradually stir in cheddar and 1/2 cup (125 mL) Parmesan until smooth. Season with nutmeg, hot sauce, salt and pepper. Whisk in yogurt until mixture is smooth.