Put the potatoes in a pan of cold water and bring to the boil. Simmer for 10-15 minutes, until tender. Drain.
Meanwhile, heat a griddle pan or barbecue to high. Brush the steak with a little of the olive oil and cook for 1½ minutes each side for medium-rare, or longer if you prefer your meat well done. Transfer to a plate and leave to rest for 10 minutes before slicing. Meanwhile, toss the courgette slices in the remaining olive oil and griddle for 2-3 minutes each side.
In a small bowl, whisk together the crème fraîche, horseradish sauce, lemon juice and mint.
To serve, toss together the potatoes, watercress, radishes, griddled courgettes and steak. Tip onto a serving platter and drizzle with the creamy dressing.