Preheat the oven to 150°C.
Place halved tomatoes on a tray, drizzle with oil, season, and scatter thyme.
Roast tomatoes for 4-6 hours, then cool completely in the switched-off oven.
Preheat the oven to 200°C for the tart.
Heat oil in a 25cm ovenproof pan and arrange sliced potatoes in overlapping circles.
Season potatoes and cook on low heat for five minutes until golden at the edges.
Top potatoes with sardine fillets in a fan shape and roasted tomatoes.
Sprinkle with oregano and bake for 15-20 minutes until potatoes are cooked.
Blanch basil in boiling water for one minute, then refresh in ice water.
Blend basil with 200ml olive oil and strain through a sieve.
Toss rocket leaves with basil oil and serve on top of the tart.