Heat the oven to 200°C/Fan 180°C/Gas 6. Warm a drizzle of oil in a deep frying pan or wok. Add the sausages. Cook over a medium-high heat for 8-10 mins. Turn the sausages every so often so they brown evenly.
Rinse and trim the leek (check out our handy tip). Slice it. Rinse and pat dry the parsley. Pick the leaves from the sprigs. Finely chop the leaves. Peel the potatoes. Thinly slice them. Toss the potatoes with a drizzle of olive oil in a bowl. Season.
Pop the browned sausages on a plate. Add the leeks to the pan. Fry for 5 mins over a low heat till soft. Drain the beans. Rinse them.
Add the chopped tomatoes and beans to the pan. Stir in 1 tbsp Dijon mustard. Return the sausages to the pan. Bring to a simmer. Season with salt and pepper. Take off the heat.
Stir the parsley into the sausage and bean mixture. Taste and adjust the seasoning. Spoon into a baking dish. Layer the sliced potatoes over the top of the sausages.
Cover the dish with foil. Bake for 25 mins till the potatoes are soft when prodded with a fork.
When the potatoes are soft, take off the foil. Bake for 15 more mins to brown the top. Serve the hotpot on warmed plates.