County Clare Irish Root Soup

Source: County Clare, Milwaukee, Wisconsin

2 tablespoons olive oil
1 tablespoon butter
2 pounds carrots, peeled and sliced
2 leeks (white part only), sliced
6 cloves garlic, peeled and chopped
2 pounds sweet potatoes, peeled, halved,
    boiled and oven-roasted (see note)
4 cups chicken stock
2 cups whipping cream plus whipped cream for garnish
Pinch of salt
Pinch of white pepper
2 tablespoons granulated sugar

Heat oil and butter in large, heavy saucepan over medium heat. Add carrots, leeks, garlic and sweet potatoes. Saut