Pour the wine and oil into a non-reactive bowl and add the balsamic vinegar, onion, garlic and oregano. Season with salt and pepper and chilli flakes.
Cut the steak into thick fingers across the grain and add to the marinade. Cover and leave to marinate overnight in the fridge.
The next day, heat a dry cast-iron frying pan over a high heat until it’s as hot as you can get it, and sear one side of each slice of bread. Rub the seared side with a clove of garlic if you like.
Lift the meat out of the marinade and pat dry with paper towels, then sear on all sides in the hot pan, rolling and shaking to ensure an even char, until the a core reaches 54ºC (129ºF) on a probe thermometer. Remove from the pan and set aside rest.
Pour the marinade and onions into the hot pan and scrape up any baked-on meat juices. Reduce the heat to a vigorous simmer and cook until everything is well reduced and the onions have softened. Add the cream, if using, and allow to bubble.
Toss the rested meat into the sauce along with any juices.
Spread the grilled sides of the bread with mustard or horseradish (I go for one of each), then spoon the meat and onions on top.
Add a little more of the sauce, spread over the lettuce leaves and nail on the lid with cocktail sticks. Serve with any leftover sauce for dipping.
