Heat a griddle pan until smoking hot. Drizzle the steak with olive oil and season well on both sides. Fry for 3 minutes each side for rare; 4-5 minutes for medium; or 6-7 minutes for well-done. Set aside, covered with foil, to rest for 5 minutes.
Mix the wasabi paste, mayonnaise and a dash of lime juice together and season with black pepper and set aside. (Add more or less wasabi paste according to how hot you like it.)
Pour the vegetable oil into a small frying pan to the depth of 1cm and heat until a small cube of bread turns golden brown in 30 seconds. Add the onions and fry for 30 seconds - 1 minute or until golden and crisp. Remove with a slotted spoon and drain on kitchen paper.
Spread each slice of toast with a generous spoonful of wasabi mayonnaise and lay the tomato slices on top. Remove any fat from the steak and thinly slice and divide between the two sandwiches.
Top with a little watercress and then scatter with the crispy onions, drizzle with a little olive oil and serve straight away.
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