If you have time, soak the onion slices in cold water for 1 hour: it helps to cut their pungency.
Drain the onions and pat them dry. Warm the baguette or rolls very gently. Split them in half and spread lightly with the low-fat spread, if using, then fill with the lettuce and tomato.
Heat the grill, or a ridged, iron grill pan, until it is piping hot. Trim any fat off the steaks, brush them with just a little oil and grill or fry them for 2-3 minutes on each side until cooked to your liking.
Place the steaks in the sandwiches, brush them with mustard or horseradish and top with the onion slices. Season to taste.
Press the sandwiches firmly to seal, cut on the diagonal and eat them immediately, wrapped in paper napkins.