Heat the oil in a nonstick wok or large deep skillet over high heat until a drop of water sizzles in it. Add the cabbage and carrots; stir fry until crisp tender, about 3 minutes. Add the mushrooms, scallions and garlic; stir fry until softened, about 4 minutes. Add the hoisin sauce and stir fry until heated through about 30 seconds.
Top each tortilla with half the mixture and if desired 1 teaspoon sour cream. Roll up to enclose the filling.