In a 2 quart / 2 L saucepan, heat the oil. Add the flour and cook, stirring for a minute over low heat. Add the curry powder and cook another minute. Whisk in the hot vegetable stock all at once: it will thicken almost instantaneously. Add the soy sauce and sweetener and cook until thick enough to coat the back of a wooden spoon.
Meanwhile, cook the udon in a large pot of boiling water. When al dente, drain well, rinse and place into 4 large bowls. Fill each bowl with curry sauce, top with scallions and serve immediately.