To cook the mussels heat up a little olive oil in a pan with 1/2 a diced onion, carrot, celery and a little tarragon. Add the white wine and stock, bring to the boil and simmer for a few minutes. Prepare the mussels and add to the boiling stock mixture. As soon as the mussels begin to open drain thm off into a bowl keeping the liquid separate. Warm a pan on the stove with some butter and olive oil and dice all the remaining vegetables. Add these to the pan and cover with the stock, bring to the boil, gently simmer adding the remaining stock if needed. Simmer until the vegetables are tender and season well with salt and pepper, add the pesto and leave to one side. Pan fry the halibut in half olive oil and butter and colour well on one side. This should only take about 3 minutes. Add the tomato to the stew and the mussels and place in the serving bowl with the halibut on the top. Drizzle with olive oil and serve. Per serving: 521 Calories (kcal); 2g Total Fat; (5% calories from fat); 8g Protein; 60g Carbohydrate; 0mg Cholesterol; 172mg Sodium Food Exchanges: 0 Grain(Starch); 0 Lean Meat; 10 1/2 Vegetable; 0 Fruit; 0 Fat; 0 Other Carbohydrates Converted by MM_Buster v2.0n.