Heat the oven to 190°C. Lightly grease 24 (1 1/2-inch) mini muffin-pan cups.
Heat olive oil in a pot. Add the onion and season with salt and pepper. Cook over low heat, stirring frequently for 10-15 minutes. When onions have softened and look golden, add sugar and balsamic to start the caramelisation process. Cook onion over low heat for a further 5-10 minutes, stirring occasionally, until sticky and caramelised.
Add sun-dried tomatoes and chorizo to the pot and combine the mixture. Turn off the heat.
Place 1 pastry sheet on a clean surface. Cut into 9 squares. Repeat with the other pastry sheets until you have 24. Press the pastry squares into the muffin-pan cups and brush the pastries with the egg.
Scoop 1 tablespoon of the mixture in the centre of each muffin pan cup.
Bake the tartlets in the oven until golden and crispy, for about 10-15 minutes. Season to taste and enjoy!!