The "roux" mentioned in the translation is probably an incorrect translation. Roux was not used as a thickener in Roman times. I have suggested the substitution of either rice flour or bread crumbs, but I have found that with this recipe, it is usually not necessary to thicken the sauce at all. By the time the carrots and parsnips are sufficiently cooked, the liquid has reduced itself to a thick paste that binds itself to the vegetables. In fact, if the pan becomes too dry, you may need to add some additional liquid to the pot to keep the vegetables from sticking.
Boil the parsnips hard, put them in a sauce pan and stew with oil, stock, pepper, raisin wine, strain, and bind with roux.
Carrots and parsnips are fried with a wine sauce.
Apicius, 119, 122; Arwen Southernwood, The Stewpot Period Culinary Guild (SCA)