Spaghetti with tuna and olives

Cook spaghetti in large saucepan of salted boiling water until al dente. Drain and keep warm. Meanwhile heat a non-stick frying pan over medium-high heat. Spray with oil. Add onion and cook, stirring for 5 minutes or until soft. Add tomato and stock and bring to the boil. Taste and season with salt and pepper. Add tuna and olives. Reduce heat to low and simmer for 5 minutes. Stir in parsley and spoon over spaghetti. Toss lettuce with dressing and serve with spaghetti.