Mash garlic, tomato, chilli, coconut, turmeric, and lime juice into a paste using a pestle and mortar.
Rub the tuna with oil and season with salt and pepper.
Sear the tuna in a hot griddle pan for 2-3 minutes on each side.
Remove the tuna and set aside to rest for a few minutes.
Combine the cabbage and red onion in a medium bowl.
Whisk fish sauce, vinegar, sugar, coconut milk, and lime juice together then pour over the salad.
Mix the fresh herbs into the cabbage salad.
Place a portion of salad and a tuna steak onto each banana leaf.
Spoon chutney over the tuna, fold the leaf into a cone, and secure with a stick.