Place the steak, onion, mustard and coriander in a food processor with a half a teaspoon of salt and black pepper to taste. Blend until combined, being careful not to over-process. Shape the mixture into six burgers. Place on a plate, then cover with clingfilm and place in the fridge until needed (this helps them to firm up).
Preheat a griddle or frying pan for about 4 minutes on a high heat, then turn the heat down to medium. Brush the olive oil on the heated pan and add the burgers. Use a spatula to lightly press down on them, making sure each burger is in full contact with the griddle. Cook them to your liking for around 3-4 minutes on each side – you may need to cook them in two batches.
To make the guacamole, cut the avocado in half, scoop out the flesh and mash with a fork. Mix in the garlic, lime juice, tomato, Tabasco and season lightly.
When the burgers are cooked, transfer to another plate and wipe the griddle pan clean with kitchen paper. Halve your white rolls and lightly toast. Arrange the bottom halves on warmed plates, add a smear of mayonnaise, then top with the lettuce, tomato slices and red onion. Cover each one with a burger and add a good dollop of the guacamole. Scatter over the remaining coriander and place on the roll tops to serve.