Preheat the oven to 200°C (400°F) (gas 6).
Put the potatoes in a medium-sized roasting pan. Add the oil, mustard, and seasoning and mix together until the potatoes are coated.
Roast the potatoes for 45 minutes or until they are tender.
Then add the cherry tomatoes and return to the oven for a further 5 minutes until their skins begin to burst.