I found this on the Epicurious website, and my husband loved it served with buttered garlic noodles and steamed green beans. The original recipe called for Pollock or Pacific cod, but I always have Tilapia on hand for this easy dish.
Put leeks in a heavy skillet with butter, 1/3 cup water, salt and pepper. Cover skillet with a tight fitting lid and simmer over medium heat, stirring once or twice, until leeks are tender, about 10 minutes.
Stir in sour cream mixture and remaining 1/3 cup water.
Season fish with salt and pepper and add to leeks and cook, covered, until just cooked through, 8 to 10 minutes.