Mix together a splash of sherry vinegar, a tablespoon of oil and cracked black pepper and rub this mixture over the steaks. Set aside while you cook the onions.
Heat a tablespoon of oil and a knob of butter in a large frying pan over a medium heat. Sauté the onions until softened, this will take about 15 minutes.
Add in a tablespoon of the sherry vinegar and a little salt and continue to cook for another five minutes until the vinegar has evaporated. Set aside.
Heat a griddle pan until really hot. Season the steaks with salt. Place them two at a time on the pan and cook for two minutes on each side for rare or until cooked to your liking.
While the steaks are cooking, in a small bowl mix together the crème fraîche, horseradish sauce, mustard and white wine vinegar/lemon juice.
Brush the bread with a little oil on each side and toast under the grill.
Spread a little horseradish cream over each slice, then spoon over the onion mixture, slice the steak and lay on top. Arrange some rocket leaves on top and serve.