Heat your oven to 200°C/Fan 180°C/Gas 6. Chop the sweet potatoes into fine slices, leaving the skin on. Cut the slices into thin matchsticks (see our tip for making extra thin fries with a spiraliser or julienne peeler).
Place the sweet potatoes 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 in a thin layer 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, tip the peppercorns into a pestle and mortar and roughly crush them, or use a small bowl and the bottom of a jam jar to crush them. Pat the steaks dry with kitchen paper. Put on a plate. Rub 1 tsp olive oil into each steak. Pat the peppercorns into both sides of the steaks.
Roughly chop half the watercress and put it in a large bowl. Zest and juice the lime into the bowl. Peel and grate in 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.
Pop the remaining watercress in a separate bowl. Slice the skin and white pith off the orange and slice it into segments, Add them to the bowl. Put a large frying pan over a medium heat and tip in the pine nuts. Toast for 2-3 mins till golden and add to the watercress salad. Set aside.
When the potatoes are 5 mins away from being cooked, put the frying pan back on a high heat for 2 mins. Add the steaks.
Cook the steaks for 1 min on each side for rare, 2-3 mins on each side for more well done steaks. 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. Serve with the watercress salad and the mustard.