Line a 9 x 17 cm (3½ x 6¾ inch) terrine or loaf tin with the Serrano ham, leaving plenty of overhang so that the terrine can be completely enclosed, then chop the remaining slices.
Heat the oil in a frying pan, add the onion and chopped ham and cook for 5 minutes until the onion is softened. Add the garlic and cook for a further 30 seconds, then pour in the sherry and cook until evaporated. Transfer to a bowl and leave to cool, then mix together with the pork and thyme and season well.
Spoon half the pork mixture into the terrine, pressing down firmly. Arrange the chicken slices on top, ensuring the pork is completely covered. Top with the remaining pork and press down well, then cover with the ham.
Cover with foil and place in a deep roasting tin. Pour in boiling water to come halfway up the sides of the tin. Place in a preheated oven, 180°C (350°F), Gas Mark 4, for 1 hour until the juices run clear.
Leave to cool slightly, then drain away any juices from the terrine. Cover the top with foil, then cut out a piece of cardboard and fit on top. Weigh down with heavy cans and chill overnight. Bring up to room temperature, remove the cans, cardboard and foil and then turn out on to a serving plate. Cut into thick slices and serve with Spanish Tomato Salad (see below), if liked.