For the chips, spread the potato wedges out in a large roasting tin and drizzle with the oil. Toss well to lightly coat the potatoes in the oil, then scatter over the salt and mustard powder and toss again. Place in a preheated oven, 220°C (425°F), Gas Mark 7, for 20 minutes.
Meanwhile, melt the butter in a large, heavy-based frying pan and cook the beef briefly over a high heat, turning frequently, until browned all over and sealed. Transfer to a work surface. Mix the mustards and herbs together and spread over the beef. Place the beef in the roasting tin with the potatoes if there is room, or in a separate roasting tin, and cook in the oven for 15 minutes until cooked through but still pink in the centre.
Slice the beef into thick or thin slices and serve with the chips tossed in the chopped parsley, accompanied by a simple salad, if liked.