Heat the oil in a large, deep frying pan (or wok) and sauté the onions for 8-10 minutes until softened. Stir in the thyme, seasoning and tomatoes and cook over a low heat for 20 minutes until golden and caramelised.
Heat the oven to Gas Mark 7, 220ºC, fan 200ºC. Line a 30x22cm (12x81⁄2in) Swiss roll tin with the pastry. Cover with the onion mix.
Arrange the anchovies over the onions in a criss-cross pattern. Place an olive inside each diamond and bake for 15-20 minutes.
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