Prepare the Oven
Preheat your oven to 200°C (fan 180°C, gas mark 6).
Cook the Sausages
Heat the oil in a large frying pan. Using kitchen scissors, cut the sausages into bite-sized pieces (4-5 per sausage). Sear them over high heat until browned, approximately 5 minutes. Transfer the cooked sausage pieces to a 23cm baking dish.
Soften the Leeks
In the same pan, add 1 tablespoon of butter. Stir in the leeks, followed by 50ml of the chicken stock and a pinch of seasoning. Cover the pan and cook over medium heat for 5-7 minutes, allowing the leeks to soften.
Prepare the Sauce
Melt the remaining 20g of butter in a separate saucepan. Add the flour and cook for 1 minute while stirring continuously. Gradually whisk in the rest of the chicken stock until a smooth consistency is achieved. Incorporate the grain mustard, thyme, and crème fraîche. Simmer gently for 2 minutes before stirring in the frozen peas and cooked leeks. Adjust seasoning as needed.
Assemble the Dish
Pour the sauce mixture into the baking dish, ensuring the sausage pieces are well-coated. Stir gently to combine. Scatter the gnocchi evenly across the top, straight from the package, and sprinkle with the grated cheese.
Bake to Perfection
Place the dish on a baking tray and bake for 25 minutes, or until the gnocchi topping turns golden and crispy.
Optional Preparation Tip
Prepare the dish up to the end of Step 5 a day in advance. Store it covered in the refrigerator, and allow an additional 5-10 minutes of baking time when cooking from chilled.