Cut dark green parts off the leeks and discard. Remove any damaged outer leaves. Cut leeks in half lengthwise and slice into 1/2-inch strips on the diagonal. Transfer to a colander. Separate slices and rinse thoroughly to remove dirt. Drain.
Place leeks in a pot over medium-high heat. Pour in Chardonnay and sugar. Cook, stirring often, until leeks are soft and tender and wine has evaporated, about 10 minutes. Remove from heat. Stir in creme fraiche and season with salt and pepper.