Heat oven to 200C/180C fan/gas 6. Put the potato wedges on a baking tray and rub with the oil. Sprinkle the sesame seeds and some seasoning over. Bake for 25 mins or until crisp at the edges.
In a small bowl, mix together the miso, lemon juice, chilli sauce and mirin. Rub the steaks with a tiny bit of oil and some seasoning. Spoon 1 tbsp of the sauce over each steak and rub into both sides.
Heat a griddle pan until really hot, cook the steaks for 2 mins each side, or longer if you prefer it well done. Brush more of the sauce over after you turn them. Transfer to a plate, cover loosely with foil, and leave to rest for 5 mins. Serve the steaks sliced, with extra sauce, the potatoes and watercress.