Heat the butter and half the oil in a frying pan. Add the mushrooms and fry over a high heat for 2-3 minutes. Remove with a slotted spoon; set aside. Add the leek and spring onions to the pan with the rest of the oil. Cook over a medium heat for 8-10 minutes.
Meanwhile, cook the potatoes in a pan of boiling water for 10-12 minutes. Drain and keep warm.
Preheat the grill to high and grill the gammon for 4 minutes on each side until cooked through.
Add the mustard and stock to the pan with the cooked onions. Stir in the soured cream and simmer for 1 minute. Return the mushrooms to the pan, add the parsley and season with freshly ground black pepper.
Meanwhile, cook the peas in a pan of boiling water for 4 minutes. Drain and serve with the potatoes and the gammon steaks topped with the sauce.