Pre-heat oven to 200°C / 400°F / Gas Mark 7.
In a wok or good-sized saucepan, heat oil and sauté onion. Add carrot, parsnip and pepper and stir-fry over a high heat to colour edges. Transfer to an ovenware dish.
Add seasoning, herbs, capers, juice and water. Stir.
Meanwhile peel the three potatoes and cut into small chunks. Cook in boiling water for approx 10mins until soft - the smaller the pieces the shorter the cooking time. Drain and mash together.
Spread mash over vegetables. Fluff with a fork, bake in oven for 10 minutes and serve.