Before you start, preheat oven to 220°C (or 200°C fan bake).
Prep potatoes, beetroot and parsnip and toss on a lined oven tray with oil. Season with salt and pepper and roast in oven for about 30 minutes, until tender and golden.
When vegetables have about 10 minutes cook time remaining, cook lamb. Heat a dry fry-pan on high heat. Add lamb and cook for 1-2 minutes each side (depending on thickness) for medium-rare, or until cooked to your liking. Rest, covered, for about 2 minutes before slicing.
Serve vegetables topped with lamb and watercress Béarnaise.