Preheat oven to 220ºC, (200ºC for fan assisted), 425ºF, Gas Mark 7.
Melt butter with olive oil in a large frying pan and sauté the onions until they are a golden brown. Add a few sprigs of thyme towards, season and then allow to cool.
Meanwhile, roll out pastry to a rectangle roughly 30cm x 20cm / 12 x 8’. Cut out 4 small rectangles and place on a lined baking sheet and score a border around the edges.
Divide the cooled onion between the rectangles and spread to within the borders, and top with some thinly sliced tomato. Then arrange the anchovy fillets on top in a criss-cross pattern and finally dot the olives amongst the anchovies and scatter more thyme leaves over the top.
Add the anchovy fillets on top in a criss-cross pattern, dot the olives amongst the anchovies and scatter more thyme leaves over the top.
Brush the borders with beaten egg and bake for approx. 15 minutes until pastry is risen and golden brown. Serve warm or cold.