Heat the oven to 200°C.
Slice the onion. Peel and slice the sweet potato and parsnip into 5mm thick circles. Finely chop the garlic.
Put the potatoes in a pan of cold water, bring to the boil and cook for 5 minutes. Drain and rinse under cold water. Slice them into thin circles.
Heat half of the oil in a roasting tin, season the steaks, then brown them on both sides and set aside.
Toss the vegetables with the remaining oil and half of the rosemary leaves - layer these in the tin, then put the steaks on top. Brush the steaks with the mustard and scatter with the remaining rosemary leaves and chopped garlic.