How can I reduce the fat in a white wine and parsley sauce?
Thickening with starch or flour instead of roux can be quite acceptable if some other rich flavors are added. Here is where the calories can be traded in: 

Good low-fat broth for part of the liquid. celery root or coriander root are particularly good ing. for veggie broth

Puree, in small amounts add body and flavor. soft cooked onion is particularly silky and surprisingly mild

Flavorsome oils, few drops instead of copious butter. Macadamia punches above its weight. Sesame toasted (tiny amount) or if feeling adventurous, pricklyash from Asian market.

lower-fat 'milks' can add luscious body instead of cream or 3% dairy milk. Unsweetened Almond or coconut doesn't overwhelm most sauces.