Coat the steak or oyster mushrooms with the fish sauce and cracked black pepper. Set aside for 2–5 minutes.
In a pestle and mortar, pound the garlic, coriander root and chillies into a rough paste. Add the fish sauce, lime juice and palm sugar, stirring until dissolved.
Heat a pan until hot.
Grill the steak to your liking, rest for 2 minutes, then slice thinly.
Or cook the oyster mushrooms until lightly charred and tender.
Combine the cucumber, tomato, red onion, celery and spring onion in a bowl. Add the warm beef or mushrooms.
Spoon over the dressing and toss lightly to coat.
Finish with chopped coriander.