Heat butter in heavy sauce pan over medium-low heat. Sautee onion and garlic until onion is translucent. Add flour and stir constantly for 20-30 seconds. Slowly add skim milk. Wisk if lumps develop. Reduce heat so it doesn't simmer. Add herbs. Slowly at cheese. When cheese is melted, simmer for a few seconds to thicken then remove from heat. When sauce has slightly cooled, stir in sour cream. (if you do use the non-fat, I suggest letting it get almost to room temperature before adding. It breakes up easier than the regular sour cream. Also, the strained yogurt would probably hold up well.) Toss with your favorite pasta.