Prepare the Oven
Preheat the oven to 200°C (fan 180°C, gas mark 6).
Cook the Sausages
Warm the oil in a large frying pan over high heat. Using kitchen scissors, cut the chicken sausages into bite-sized segments (approximately 4–5 pieces per sausage). Brown them in the hot oil for around 5 minutes until they are golden and cooked through. Once done, transfer the sausage pieces into a 23 cm ovenproof baking dish.
Soften the Leeks
In the same pan, melt 1 tablespoon of butter. Add the sliced leeks, followed by 50 ml of the prepared chicken stock and a small amount of seasoning. Cover the pan with a lid and allow the leeks to cook gently over medium heat for 5–7 minutes, until tender.
Prepare the Sauce
In a separate saucepan, melt the remaining 20 g of butter over medium heat. Stir in the flour and cook for 1 minute, stirring constantly to form a roux. Gradually whisk in the remaining chicken stock, ensuring a smooth, lump-free mixture. Incorporate the grain mustard, chopped thyme, and crème fraîche. Let the sauce simmer for 2 minutes, then fold in the frozen peas and softened leeks. Adjust seasoning to taste.
Assemble the Dish
Pour the prepared sauce into the baking dish over the browned sausages, ensuring even distribution. Stir gently to combine. Arrange the gnocchi across the surface straight from the package, then top with the grated cheese.
Bake to Perfection
Place the dish on a baking tray and transfer it to the preheated oven. Bake for 25 minutes, or until the gnocchi are crisped and golden on top.
Optional Preparation Tip
This dish can be partially prepared in advance by completing steps up to the end of the leek cooking (Step 5). Store covered in the refrigerator and allow an extra 5–10 minutes baking time if cooking from chilled.