Rinse the potatoes in cold water then place in a pan of salted water. Bring to a simmer and partly cook- they should still be a little firm in the centre. It usually takes 5-8 minutes. Drain and spread the potatoes out on a tray to cool for at least an hour.
Make the sauce. Heat a glug of olive oil in a medium saucepan over a medium heat. Add the onion, garlic, chilli, smoked paprika and cumin seeds. Cook, stirring frequently, for 4 minutes or until the onion is very soft.
Add the tinned and fresh tomatoes and the vinegar. Bring to the boil, then simmer gently for about an hour until the sauce has reduced by half, stirring from time to time. Season well and stir in the thyme leaves plus a splash of oil. Remove from the heat.
Preheat the oven to 200 C, !80 Fan, Gas 6. Tip the potatoes into a roasting tray and liberally drizzle with olive oil. Toss to coat. Season well.
Roast, shaking the tray occasionally, for 45 minutes or until the potatoes are crisp and golden brown. Sprinkle the potatoes with sea salt and spoon the bravas sauce on top. Serve with Alioli on the side.