Peel the carrots and cut them into matchstick pieces, about 2" (5cm) long and ¼" (5mm) thick. Peel and finely chop the onion. grate (shred) the rind and squeeze the juice of the orange.
Heat the oil in a large frying pan or wok, add the carrots and onion and sauté over a high heat for five minutes, stirring regularly. Stir in the orange rind and juice, and add the mustard. Add salt and freshly ground pepper to taste and simmer gently for 4-5 minutes until the orange juice is reduced to a syrupy glaze. Garnish with parsley sprigs and serve hot.