Heat your oven to 200°C/Fan 180°C/Gas 6. Chop the potatoes and sweet potato into fine slices, leaving the skin on. Cut the slices into matchsticks (see our tip for making curly fries with a spiraliser or julienne peeler).
Place the sliced potatoes and sweet potato in a large bowl. Pour in 1 tbsp oil and toss together with plenty of salt and pepper. Peel and slice the onion into thin rings. Add to the bowl.
Spread the onions and potatoes thinly on a baking tray (the more spread out they are, the crispier they’ll be). Cook for 20-25 mins till golden and crispy, tossing with a fork every 10 mins to make sure they cook evenly.
Meanwhile, remove the steaks from their packaging and pat dry with kitchen paper. Put on a plate. Rub 1 tsp olive oil into each steak. Grind over plenty of black pepper. Set aside to come up to room temperature.
Roughly chop half the watercress and put in a large bowl. Zest and juice the mandarins into the bowl. Peel and grate the garlic. Add this too. Drizzle in 1 tbsp olive oil and whisk with salt and pepper to make a loose watercress dressing. Set aside.
When the potatoes are 5 mins away from being cooked, heat a large griddle or frying pan for a few mins. When smoking hot, add the steaks.
Cook the steaks for 1 min on each side for rare, 2 mins on each side for medium and 3 mins on each side for well done. Remove from the pan and rest for 2 mins. Pile the shoestring potatoes on 2 serving plates. Add a steak and spoon over the dressing. Garnish with the remaining watercress sprigs.