Preheat the oven to 190C/375F. Heat the oil in a saucepan or a frying pan with a lid. Add the Spanish onions, leeks and red onion and stir until coated with oil. Cover and cook very slowly and gently for 1 hour. Season generously with salt and pepper. Increase the heat, add the wine, vinegar and lemon juice. Stir until the wine has almost disappeared. Remove from the heat.

While the onions are cooking, roll out the pastry on a floured surface and use a saucer to cut out four 10 cm / 4 inch circles. Prick the bases all over with a fork. When the onions have been cooking for about 40 minutes, put a baking tray in the oven for 1 minute to heat. Place the circles on the baking tray and bake for 10-15 minutes until golden and puffy. Turn the circles over, spoon some sticky caramelised onions on to each one and bake for a further 5 minutes.

Put the tarts on 4 warm plates, scatter fresh chives over them and serve.