Sear the meat in hot oil in portions. Season with salt and pepper, dust with flour and sauté briefly. Pour in the stock and cook covered for approx. 1 hour.
In the meantime, peel the cucumber, cut in half lengthways, core and cut into thick slices. Clean the spring onions and cut into rings. Melt the butter in a saucepan, fry the cucumber and onions until translucent. Pour in the cream, cook for about 10 minutes. Only stir in mustard now so that it doesn`t lose too much of its aroma during cooking. Season to taste with salt, pepper, lemon zest and dill. Mix the cucumber in the cream with the meat and serve with boiled potatoes.