Preheat the oven to 180 °C /hot air160 °C /. Make the potatoes about 7-10 min until they start to soften. Drain and return to the hot saucepan and shake heartily until the liquid has evaporated.
Heat 4 tbsp olive oil in an ovenproof baking dish, add the potatoes and parsnips, season and mix well until coated with the oil. Place in the oven and bake for 50 min. Mix the remaining tbsp of oil with the coarse-grained mustard. Pour over the potatoes and parsnips and stir until the whole is coated with the mustard mixture. Continue roasting in the oven for another 5 to 10 min until the potatoes are crispy.