First make the tomato sauce. Heat oil in a pan and fry onion and garlic over gentle heat until softened but not brown. Pour over Cirio chopped tomatoes and sugar. Bring to bubbling and reduce slightly. Season with salt and freshly ground black pepper. Cool.
Preheat the oven to 200 C / Gas 6.
Place pastry on a worktop and unroll flat. Using a saucer as a template, cut out 4 circles. Discard extra pastry. Spread over the cooled tomato sauce, leaving a rim of 1cm (1/2 in). Top with mozzarella and finish with a slice of salami. Brush egg wash over the exposed edge of the pastry.
Bake in the preheated oven until pastry is crisp, airy and golden, about 10 minutes. Serve warm scattered with ripped oregano leaves. Garnish with watercress.