Trim any fat or sinew from the venison and place in plastic bag with the teriyaki marinade and sesame oil, turn to coat the steaks in the marinade. Seal the bag and refrigerate for at least two hours, turning occasionally.
Mix the mayonnaise with the mustard and season to taste.
If making the caramelised onions, melt the butter in a frying pan and add the onions. Stir to coat with the butter, add two tablespoons water, cover and cook over a gentle heat for ten minutes.
Uncover, sprinkle with the caster sugar and balsamic vinegar and turn up the heat. Cook over a brisk heat for about another ten minutes, stirring from time to time, but watching it like a hawk! The onions should start to turn a beautiful rich brown colour, if not, just cook a little longer! Set aside when cooked.
Heat a ridged griddle pan until smoking hot. Remove the venison from the marinade and pat dry with kitchen paper. Put the venison on the griddle to sear for 2-3 minutes then turn the heat down to medium and griddle without moving for another 2-3 minutes. Turn over and cook for a further three minutes. This is for rare meat, cook for 2-3 minutes longer for medium rare. Lift onto a plate, cover with foil and leave to rest for ten minutes to allow the venison to relax.
Brush the ciabatta loaf and with olive oil then toast on the griddle or under the grill.
Slice the Venison thinly. Spread one half of the toasted ciabatta liberally with mustard mayonnaise, top with a bit of salad, then with the venison, then the onions (if using) more mayonnaise and finally the remaining ciabatta top. Cut the bread in two or four and eat immediately.