Marinated Carrots

3 lbs. carrots
1 medium onion, sliced and separated into rings
3/4 cup cider vinegar
2/3 cup sugar
1/2 cup vegetable oil
1 teaspoon salt
1/4 teaspoon pepper
1 teaspoon prepared mustard
1/4 cup snipped parsley

Peel and cut the carrots diagonally into 1/4 inch slices. Cook in boiling, salted water until crisp-tender, 6-8 minutes; drain. Combine the remaining ingredients, stirring until sugar and salt are dissolved. Pour over warm carrots. Refrigerate, covered or in a Tupperware, at least 12 hours. Serve with slotted spoon.