Marinate the steak in the oyster sauce, soy sauce, sugar and oil while you prepare the rest of the recipe.
Make the sauce next by thinly slicing the spring onions and finely chopping the coriander stalks.
Blend the coriander together with the lime juice, fish sauce, soft brown sugar, chilli flakes and rice flour and put into a dipping bowl with half the spring onions, saving the other half for the salad.
Now work on the salad. Cut each baby gem lettuce into 6 wedges.
In a large bowl, whisk together the sesame oil and the rice vinegar with a pinch of salt to make a salad dressing.
Add in the baby gem wedges, herbs and the rest of the spring onions. Toss together until the leaves are fully coated and scatter with the toasted sesame seeds.
Heat a griddle pan or heavy based frying pan over a high heat. Sear and cook the steak for 2-3 minutes on each side.
Allow to rest for 5 minutes before slicing.
Serve with the salad and the dipping sauce.
Notes
Tri Tip (or triangle steak) comes from the bottom of the sirloin of the animal, as such it has more marbling and a denser texture than sirloin steaks which come from the very top of the sirloin. It can be tricky to get hold of, especially outside the US so if you struggle to find it, use well marbled thick cut boneless rib eye instead. You can make your own toasted rice flour which is as straightforward as it sounds. Simply roast a handful of sticky or other short grain rice or toast in a pan and then grind to a powder.