Bring a large pot of salted water to a boil. Add carrots and cook until tender but still firm. Drain, cool and grate into a large bowl.
In a small saucepan, whisk together the tomato sauce, oil, sugar, vinegar, mustard and Worcestershire sauce. Bring to a boil, whisking constantly, remove from heat and cool.
Add onions and mint leaves to the carrots, and pour the dressing over all. Toss until evenly coated and chill for 24 hours before serving.
