Trim beef steaks of excess fat, then finely dice the meat. Combine with garlic, ginger and 2 tablespoons of the soy sauce in a bowl and leave to marinate at room temperature while you prepare the rest of the ingredients.
Mix remaining soy sauce, sesame oil and sweet chilli sauce together and set aside.
Wash the baby Cos well, then discard the outer leaves and cut 3cm off the bottom stem. Tease the leaves apart to separate them and set aside on a platter.
Thinly slice spring onions, chop coriander and finely chop chilli.
Heat oil in a wok or large fry pan on high heat. Stir-fry beef quickly until browned (1-2 minutes), then add soy sauce mixture and allow it to bubble away for 1-2 minutes while you toss the beef to coat in the sticky sauce.
To assemble, place a spoonful of teriyaki beef in each lettuce leaf and arrange on a platter. Garnish with spring onion, coriander and chilli. Serve immediately and use your hands to eat.