Remove the meat from the fridge approx. 1 hr. prior to cooking.
Heat the oil in a large frying pan. Fry the parsnips and mustard seeds for approx. 5 mins., stirring occasionally, season with salt. Pour in the water, cover and cook the vegetables for approx. 20 mins.
Mix the flour and salt in a measuring cup. Add the eggs one at a time along with the milk and oil, beat until smooth using the whisk on a hand mixer, cover and leave to stand for approx. 20 mins.
Combine the butter, mustard, salt and pepper, use the mixture to coat the meat. Insert the meat thermometer into the thickest part of the joint (fatty side up).
Pour the oil into the muffin tin holes. Place the muffin tin on a baking tray lined with baking paper. Place the tray in the lower half of an oven preheated to 240 °C for approx. 5 mins. and an oven rack in the upper half. Pour the Yorkshire pudding mixture into the holes of the muffin tin, return to the oven. Place the meat on the hot oven rack above the muffin tin. Cook for approx. 10 mins., reduce the oven temperature to 200 °C and cook for a further 15 mins. (the core temperature of the meat should be 55°C). Remove the roast beef, cover and leave to rest for approx. 10 mins. Turn off the oven and keep the Yorkshire puddings warm in the oven with the door slightly ajar.
Cut the roast beef into thin slices, serve with the vegetables and Yorkshire puddings.