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Heat the oven to 140C/120C fan/gas 1. Put the tomatoes, cut-side up, on a large baking sheet lined with baking parchment. Sprinkle over 1 tsp sea salt, drizzle with 1 tbsp oil, and scatter over the thyme. Roast for 1½-2 hrs, or until the tomatoes are wrinkled and a touch charred but still juicy. Leave to cool.
Combine the ricotta, parmesan and lemon zest. Season. Heat a griddle pan over a high heat. Brush the bread with the remaining oil and griddle for 2-3 mins on each side, or until lightly toasted. Top with the ricotta mixture, tomatoes, prosciutto and a drizzle of olive oil.