Preheat oven to 220°C fan forced. Place potatoes in a saucepan, cover with cold water. Bring to the boil over high heat. Boil, uncovered for 20 minutes or until just cooked. Drain. Transfer to a large well-greased roasting pan. Gently press potatoes to flatten slightly. Spray with olive oil and season. Roast for 30 minutes until crisp.
When the potatoes have 20 minutes to go, place a second roasting pan under the potatoes to get hot. Whisk the oil, lemon rind, juice, mustard, sugar, garlic and salt and pepper. Add the mushrooms to the hot pan. Spoon over half the dressing and turn to coat mushrooms. Roast 15 minutes or until just tender.
Arrange the potatoes over base of a serving platter, scatter over the roasted mushrooms. Top with fresh herbs and spoon over the remaining salad dressing. Serve warm or at room temperature.