Trim the base and any woody tops off the fennel. Cut the bulb in half and slice it into thin pieces. Trim the root and top 3cm off the leek. Slice it in half and rinse under a cold tap. Thinly slice the leek. Peel the carrot and trim the ends off the celery. Finely dice them both.

Warm 1 tbsp olive oil in a large pan on a medium heat. Add the fennel, leek, carrot and celery. Season with a pinch of salt and pepper and fry for 10 mins, stirring occasionally, till softened.

Meanwhile, peel and finely chop the garlic. Drain and rinse the cannellini beans with cold water. Fill and boil the kettle.

When the veg have softened, add the garlic and cook for 1 min. Tip in the cannellini beans and the cherry tomato passata. Pour in 300ml boiling water. Add the saffron powder and crumble in the stock cube. Stir well and bring to the boil, then reduce to a gentle simmer and cook for 15 mins.

Meanwhile, pick the tarragon leaves and roughly chop them. When the stew has simmered for 15 mins, add the fish pie mix and chopped tarragon to the stew and gently stir. Cover the pan with a lid and simmer for 5 mins. The fish will be opaque and flake easily when pressed.

Juice half the lemon into the pan. Have a taste and add more salt or pepper if you think it needs it. Ladle the fish stew into a couple of warm bowls and serve with wedges of the remaining lemon half on the side.