Preheat your oven to 200°C/Fan 180°C/Gas 6. Scrub the potatoes and slice them into bite-sized chunks. Remove any leaves from the beetroot (save these for a salad or a stir-fry on another day). Scrub the beetroot. Chop the beetroot into 1cm-thick slices.
Tumble the potatoes and beetroot into a large roasting tin. Toss with 1 tbsp olive oil and a pinch of salt and pepper. Slide into the oven and roast for 20 mins.
Meanwhile, peel the red onion and slice it into thin wedges. Set aside.
Spoon half the pot of crème fraîche into a bowl. Stir in 1 tbsp horseradish mustard. Set aside.
When the beetroot and potatoes have cooked for 20 mins, scatter the onion into the roasting tin. Turn the veg over in the pan, then return to the oven for a further 15-20 mins, till tender.
While the veg roast, pour 1 tbsp olive oil into a large frying pan and warm to a high heat. Rub a little salt and pepper onto the steaks. When the pan is sizzling hot, add the steaks and fry for 30 secs-1 min per side, then transfer them to a plate and set aside to rest for 3-4 mins.
Peel and finely chop the garlic clove. When the vegetables are cooked, remove the tin from the oven and add the garlic. Pour over 1 tbsp balsamic vinegar and stir. Fold in the watercress.
Divide the potatoes, beetroot and watercress between a couple of warm plates. Dollop over the horseradish crème fraîche. Slice the steaks and arrange on top. Halve the lemon and squeeze over the juice from 1 half to serve.