Light a wood-fired barbecue and let it burn down to ensure you have a nice even heat by the time you are ready to cook (1-1½ hours).
For jaew, dry-roast chillies in a wok or frying pan over low heat until aromatic and roasted (2-3 minutes; this gives the jaew a beautiful smoky flavour, but be careful not to burn the chillies or they’ll turn bitter). Cool, then pound with a mortar and pestle or slightly crush in a spice grinder until crunchy (not to a fine powder). Combine 3 tsp crushed roasted chilli, or to taste, in a small bowl with remaining ingredients and mix well. You may need to balance the flavour with more fish sauce or sugar; it should taste hot, sour, salty and slightly sweet. Refrigerate until required.
Lightly oil steak on both sides and season to taste with sea salt. Grill, turning once, until browned and cooked to your liking (5-7 minutes each side for medium-rare), then rest for 20 minutes. Slice beef off the bone, then cut slices across the grain and arrange on a serving plate. Serve with the lettuce leaves, herbs, lime wedges, jaew and rice.