Heat butter and oil in a medium non stick frypan over low heat. Add onion and sugar, cook for approximately 10 minutes or until tender and caramelised, stirring occasionally.
Add Leggo’s Tomato Paste and cook for a further 1-2 minutes. Stir in diced tomato and herbs. Allow to cool.
Cut each pastry sheet into 12 x 6cm rounds to fit into 24 mini muffin pans. Spoon tomato mixture into pastry cases. Bake in a preheated oven at 200°C for 12 -15 minutes.
Combine mascarpone cheese, milk and basil and season to taste. Top cooked tartlets with a spoonful of mascarpone cheese. Garnish with basil and serve.