Lay the beef in a non-metallic shallow dish. Stir the garlic, oregano, chillies and some pepper into the olive oil. Pour it over the beef, rubbing it in to coat both sides. Leave to marinate for at least 10 minutes.
Brush both sides of the bread with a little extra oil. Heat a griddle pan then cook the bread on it until lightly toasted on each side. Set aside.
Lift the beef from the marinade and lay it on the hot griddle. Cook for 2-2.5 minutes on each side for medium rare, 2.5-3 minutes for medium and 3-3.5 for well done. Remove and let the meat rest for 5 minutes.
Mix the soured cream and horseradish together. Scatter some watercress sprigs onto each slice of bread, then drizzle a line of the horseradish cream mixture down the middle. Lay the sunblush peppers casually on top and scatter over a few capers. Slice the beef quite thinly, then arrange it in a line over the peppers. Season, and serve 2 slices per person.
COOK'S TIP For a more intense flavour, the beef can be marinated overnight, then quickly cooked on the griddle or barbecue when ready to serve. Just remove from the fridge 30 minutes before cooking.
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