Preheat oven to 200C. Cover potatoes with cold salted water in a large saucepan, bring to the boil over high heat and cook until just tender (8-10 minutes). Drain, halve potatoes and set aside.
Cover base of roasting pan with oil and heat in oven for 10 minutes. Add potato, onion and garlic to pan, stir to coat with oil and roast until golden and crisp (20-30 minutes), remove from oven, set aside.
For mustard dressing, whisk together oil, vinegar and mustard and season to taste. Drizzle over vegetables and set aside until cooled. Transfer to a serving bowl, toss with mint and serve immediately.