Trim off the pointed ends of the baguette and cut it into four segments; split each horizontally into two halves.
Lightly season the beef with salt and pepper and set aside for 30 minutes.
To prepare chimichurri sauce: Combine the herbs, garlic, shallots and olive oil in a food processor. Blitz for two to three seconds to get a rough,chunky paste. Stir in the rest of the ingredients, seasoning to taste. Keep sauce covered with a little oil in a glass container until needed – keeps well in the fridge for several days.
To cook steak: Heat the olive oil in a nonstick pan. Fry the marinated beef for two to three minutes on each side for medium doneness. Remove and set aside for 5 minutes before slicing thinly.
To serve: Toast the bread if desired. Heat a frying pan and cook the onion rings with the oil until just cooked through.
On the bottom half of the baguette, layer the lettuce, cucumber, capsicum and onion. Top with slices of beef. Spoon some chimichurri sauce over and top with the other half of the baguette.