Cook the potatoes in a large pan of boiling salted water for 15 minutes or until tender, then drain and set aside.
Meanwhile, heat 1tbsp of the olive oil in a frying pan, add the onion and fry on a medium heat for 5 minutes or until coloured slightly and softened. Remove from the heat and tip into a large bowl, then add the cooked potatoe
Whisk 2 tablespoons of olive oil together with the vinegar, mustard and honey, then season. Pour it over the onions and potatoes and stir together.
Heat a griddle pan until smoking hot, drizzle the steaks with the rest of the olive oil and season well. Fry for 3 minutes each side (a little longer if you prefer meduim) Cover with foil and allow to rest for 5 minutes.
When you are ready to serve, stir the watercress and the blue cheese through the onions and potatoes. Put one steak on each plate and serve immediately with the potato salad on the side.