For the tomatoes, preheat the oven to 150C/300F/Gas 2. Place the tomatoes, cut side up onto a roasting tray and drizzle with the olive oil.
Season with sea salt and freshly ground black pepper then scatter the thyme over the top. Place in the oven and roast for 4-6 hours until slightly dried out. Turn the oven off and allow to cool, preferably overnight, in the oven.
For the tart, preheat the oven to 200C/400F/Gas 6. Heat a medium sized (about 25cm/10in) ovenproof frying pan over a medium heat then add a drizzle of the olive oil.
Arrange the sliced potatoes in overlapping circles so they cover the bottom of the pan. Season with sea salt and freshly ground black pepper then turn the heat down to low.
Cook for about five minutes until the potatoes turn golden-brown at the edges and are translucent in the middle. Place the sardines on top in a fan shape, with the tails pointing towards the middle of the pan.
Lay the oven-roasted tomatoes over the top then sprinkle with the oregano. Place in the oven and bake for 15-20 minutes until the potatoes are cooked and the top is golden brown. Remove the pan from the oven and let the tart cool slightly.
For the salad, bring a pan of salted water to the boil, add the basil, blanch for one minute then drain and refresh in iced water.
Squeeze out the water and place the basil into a food processor with the olive oil. Blend for 1-2 minutes until the oil has turned vibrant green then strain through a fine sieve into a bowl. Toss the rocket leaves with a little of the oil.
To serve, place the tart onto a plate then pile the salad on top.