Put the tomatoes, cut sides up, in the base of a grill pan, drizzle with 1 tablespoon of the oil, season and cook under a preheated hot grill for 5 minutes until browned. Heat the remaining oil in a saucepan, add the onion and fry for 5 minutes until pale golden. Stir in the paprika and cook for 1 minute. Add the grilled tomatoes and crush with a wooden spoon, then mix in the sun-dried tomatoes, stock, sherry and sugar, and season to taste with salt and pepper. Bring to the boil then cover and simmer for 20 minutes.
Meanwhile, make the croûtons. Mix the oil and paprika together in a plastic bag, add the cubed bread and toss together. Tip out on to a baking sheet and cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 10 minutes or until crisp and golden.
Cool the soup slightly then purée in batches in a blender or food processor until smooth. Sieve and return to the saucepan, cover and set aside until required. When ready to serve, reheat the soup until piping hot, ladle into small bowls and sprinkle with the croûtons and a little olive oil.